DI-73 – Offshore Drilling with Light-weight Fluids



Proposal for a Joint Industry Project on

Offshore Drilling with Light-weight Fluids

PETROBRAS Research Center

Exploration & Production Division

May, 1997



1. Introduction

2. Project Objective

3. Project Modules

3.1. Reservoir Assessment

3.2. Drilling

3.3. Wellbore Stability

4. Deliverables

4.1. Reservoir Assessment and Characterization

4.2. Drilling Hydraulics

4.3. Four-phase Separator

4.4. Floating Rig Application

4.5. Wellbore Stability

5. Available Resources

5.1. Research Well Characteristics

5.2. Research Labs

6. Project Team

7. Costs and Funding


This proposal contains some topics for the development and optimization of the drilling technology with light-weight fluids, with special attention to offshore environments with floating platforms. A first draft of this proposal was already presented to some operating companies which contributed with some valuable suggestions.

It is expected that with this plan as the basis, a steering committee could be formed for more detailed discussion of each topic, including alternatives for cooperation in the development, costs and time schedule. It is important to point out that Petrobras will start the development of some of the items of this proposition in the second semester of 1997, even before the signature of formal affiliation contracts with most of the potential participants, due to the company’s urgent need.

Particularly, this technology entices PETROBRAS’ attention since most of its reserves are located offshore at large water depths. The possibility of drilling re-entry wells, including horizontal and multi-laterals, aiming at larger production rates and improved final recovery, as well as, lower installation and drilling costs, seems very attractive, specially in low pressure and depleted reservoirs.

At this stage, though, it is noticed that some aspects still require additional development for its technical and economical implementation. Considering the general interest of the industry on this subject, PETROBRAS, through its Research and Development Center (CENPES) decided to look for partners, cooperators and affiliates for this project with the objective of sharing needed resources and accelerating the achievement of practical and economical results.

1. Introduction

The increasing application of radial drilling technology, specially in low pressure fields, associated to the use of light-weight drilling fluids (foam/aerated muds), is making evident the present technical limits and the necessary improvements for higher production rates and drilling costs reduction.

In the last few years, the industry has been addressing this subject with great interest. Technology reviews and the development of equipment, materials and computer simulators through several joint industry projects and independent research groups are under way. However, some important aspects still require more efforts in the search for appropriate solutions that could improve the possibilities of success in the application of this technique. Some of these points constitute the subject of this proposal.

First of all, a sound methodology for reservoir assessment and characterization has to be defined. Up to now, many of the horizontal wells and radials do not present as high production rates as expected. This is a critical point in the feasibility analysis of tight projects in low flow rate reservoirs.

On the drilling side, the interest is focused on definition of specific drilling/safety operational procedures; hydraulics optimization with foam/aerated fluids, including static/circulating pressure predictions, cuttings transport, down-hole motors requirements; field data analysis and comparison with available simulators; experimental work in labs, flow-loops and full-scale facilities with foam/aerated fluids; separation system at surface for underbalanced operations; and, finally, the requirements for implementation of this technology offshore, in floating rigs.

At last, wellbore stability is a concern not only while drilling but, also, during the production period under variable conditions of flow rates and pressures.

2. Project Objective

The main purpose of this project is to contribute for making the light-weight fluid drilling technology a technically and economically feasible option for application in offshore environments, specially for wells drilled from floating platforms.

3. Project Modules

Three modules are devised for this project: (1) Reservoir Assessment; (2) Drilling; (3) Wellbore Stability.

3.1. Reservoir Assessment

Petroleum reservoirs often have a gas cap and/or an aquifer. In these situations they are subjected to rapid gas and/or water movement towards the well, created by a sharp pressure gradient in the well direction. As the production begins, the interface between the fluids, that is, gas-oil contact or water-oil contact, deforms from its initial plane shape to a cone or crest. When a field is developed by vertical wells, the shape of the deformation is called a cone, but when it is developed by horizontal wells, this deformation is better described as a crest.

The literature reports analytical solutions for horizontal wells for critical rate under the steady-state condition reached by the cone with a constant potential boundary . There are also analytical solutions available for infinite-acting reservoirs and for closed-boundary reservoirs, the latter being specially useful when there are several production wells in a line, creating a no-flow boundary. In those situations, it becomes important to know the breakthrough time of the displacing phase (gas or water) and the post-breakthrough behavior for supercritical rates . Some numerical solutions for this latter situation are also available.

At the beginning of a reservoir simulation study, it is a common procedure to make an estimate of the breakthrough time and the post-breakthrough behavior. Grid sensitivity runs are also made to obtain the best grid block sizes to use, since an accurate representation of cresting behavior requires fine grid simulations, which are costly and not always practical.

The focus of the work will be the development of a Visual Basic application to calculate correlations for cresting behavior in horizontal wells available in the literature for critical rate (steady-state solutions) and breakthrough time and post-breakthrough behavior for super-critical rates (transient solutions). Also, some tools to calculate an optimum grid to start a reservoir simulation study will be developed.

The tasks of this module can be divided as follows:

Selection of the correlations available in the literature.

Development of the software based on the selected correlations.

Application of the correlations in a real field example.

3.2. Drilling

3.2.1. Drilling Hydraulics and Cuttings Transport

The use of aerated fluids and foam is basically associated to the desire and need of having a low pressure profile inside the well. Applications of this type of fluids in environments with low pressure or depleted reservoirs and in hard rock drilling bring several advantages, such as smaller formation damage, higher penetration rates and reduced loss of circulation problems, just to mention the most important ones.

However, each drilling scenario demands the maintenance of the pressure profile inside an optimum range during the operations. The achievement of this goal can be accomplished by the availability of:

(1) a reliable and simple system for designing the drilling hydraulics program, involving the prediction of the equivalent circulating density for the multiphase drilling fluid, which may contain a non-Newtonian liquid, gas, oil and solid particles and;

(2) a suitable set of operational procedures for keeping the bottom hole pressure within the desirable range during connections, tripping or any other operations where the circulation is interrupted. During these periods, the multiphase system is not in equilibrium and, thus, the pressure profile has a tendency to change gradually along the time.

The system for hydraulics design will be based on available models that, for its validation, will be compared against experimental and field data, to be gathered along the project. Field data will be collected in-situ, during pre-drilling tests or during the operations, using memory or real-time pressure and temperature sensors. Experimental data will be obtained through controlled tests in a real-scale instrumented well, in flow-loops and labs available in PETROBRAS. All the experimental facilities are briefly described in section 5.

Another aspect for considerations is the analysis of solids transport by compressible fluids, which still requires much effort. The role of liquid and gas phases on the transport is not clearly defined. Basically, it is assumed that in down-hole conditions gas volume fractions are small and transport is governed by the liquid phase. In the shallow portions of the well, gas velocities are high enough to contribute to cuttings transport. Considering these assumptions, transport in the deeper portions may require attention, specially in highly inclined wells, where a cuttings bed is generally formed by gravitational segregation.

Foam and foamed liquids, due to their pseudo-plastic behavior, are known to have a higher carrying capacity than regular fluids. This may be an effective alternative for hole cleaning improvement. However, in down hole conditions, gas fractions are often below the lower limit for foam generation, resulting in poor efficiency of foaming agents in these critical regions.

With the objective of determining designing criteria for minimal hole cleaning by different kinds of light-weight drilling fluid (foam/aerated fluids) the following steps will be taken: (1) field/experimental tests for evaluation of solids return time in vertical wells, for different fluid and gas flow rates and in the presence or not of foaming agents; (2) lab evaluation of solids transport velocity by foamed fluids, considering wall and population effects; (3) lab evaluation of a solids bed erosion by foamed fluids in horizontal and highly inclined sections in a 12 m long, 5 in OD flow loop built at PETROBRAS Research Center; and (4) results of experimental work will be incorporated to a mechanistic model for cuttings transport prediction which aims to suggest guidelines for the hydraulic design of compressible drilling fluid.

3.2.2. Foam Rheology and Formation Damage

Tubular viscometers with dedicated data acquisition system and controlled flow rates, pressures and temperatures will be used for foam rheological characterization under conditions similar to those found in field operations (shear rates between 500 and 1500 s-1). Several models will be used to fit the experimental results.

The formation damage potential of the proposed foam formulations will be evaluated in a physical formation damage simulator, linked to a data acquisition and control system. The variation of formation permeability for oil, due to foam invasion, will be measured in Berea sandstone plugs under different conditions of pressures, temperatures and flow rates.

3.2.3. Four-phase Surface Separator

Since just a few large four-phase separators are available in the market, they still present high costs, which affect considerably the feasibility analysis of many onshore projects. For offshore applications, smaller and cost effective separators need to be designed.

PETROBRAS and the University of Campinas (UNICAMP) are beginning the development of a prototype of a vertical cyclone two-phase separator that will be tested by the end of 1997 at the experimental well. This prototype will be the basis for the four-phase separator design which will be designed, built and tested in 1998.

3.2.4. Operational Procedures

The operational procedures will include, primarily, a detailed manual which will compile the most important aspects associated to this type of drilling, such as equipment, safety, drilling hydraulics and down-hole pressure control during the execution of different types of operations. It will be based on proven field experience and on experimental tests at the research facilities.

3.2.5. Application in Floating Rigs

The implementation of light-fluid drilling technology in floating rigs is dependent on reliable and economical solutions for equipment compactness, flexibility and modularity, system deployment and closed-loop fluid handling system, including the four-phase separator and production facilities.

On the other hand, the precise definition of adequate operational procedures is also crucial, specially for those related to pressure profile maintenance along the well and riser during the execution of several kinds of operations such as drilling, tripping, making connections, running casing, etc.

Thus, this project will be focused on the development of a compact four-phase separator and the definition of a modular fluid handling system which could fit in different platform lay-outs. Also, based on experimental data, computer simulations and, if possible, field data, operational procedures will be elaborated and reported in a manual.

3.3. Wellbore Stability

A software for wellbore mechanical stability will be included in this project for analysis while drilling, as well as during production, since the predictions from reservoir simulators have to be verified against the stability of the producing zones as a function of the desired production flow rates. This task will be accomplished through the conversion into Windows platform of SIGMA, a numerical simulator for stability analysis used nowadays by PETROBRAS.

SIGMA is an Interactive Finite Element Code for Elasto-Plastic Static and Visco/Elasto-Plastic analysis of two and three dimensional models. The code is able to simulate the behavior of geo-technical and structural problems where the non-linear behavior of the materials involved becomes mandatory. Considering the specific application of well stability analysis, there is a subset of the code called SINESP2D that is able to simulate the behavior of the well at both drilling and production phases. The elements are automatically excavated by the code in non-linear basis. Elements can be also introduced during the analysis to represent the effect of the casing installation. It is possible to consider discontinuities in the rock mass by the application of special interface elements with different constitutive laws. The boundary conditions of the continuum can be represented by infinite elements.

SIGMA’s main characteristics include:

Gravitational initial stress condition;

Coupled pore-elastic and pore-elasto/plastic behavior, considering steady-state flow condition of a mono-phase fluid;

Visco-elasto plastic behavior;

Automatic mesh-rezoning capability with excavation and introduction of elements during linear and non-linear analyses;

Different constitutive laws for elasto-plastic and visco/elasto-plastic analyses specially developed to handle geo-technical problems;

Pre and post-processing systems coupled with the “solver”;

Graphical interface for interactive data entry and finite element mesh generation, including all the necessary data for the analysis, solver running and post-processing system call;

The discontinuities of the rock mass can be represented by special interface/contact elements with different constitutive laws;

The boundary condition of the model can be represented by special infinite elements with different decay functions.

4. Deliverables

4.1. Reservoir Assessment and Characterization

Software for evaluation of cresting behavior in horizontal wells for critical rate (steady-state solutions) and breakthrough time and post-breakthrough behavior for super critical rates (transient solutions);

Report containing an application of the software in a real field case.

4.2. Drilling Hydraulics

Drilling operational procedures manual – vs. 1.0;

Report containing field data from six different well geometries, including comparison with available hydraulics simulators;

Integrated hydraulics software for well design and field use, including two-phase steady-state analysis, down-hole motors requirements and cuttings carrying capacity – vs. 1.0;

Report of real-scale cuttings transport tests, with tentative measurement of solids flow rates both in injection and return;

Report on foam rheology and formation damage tests, including the description of equipment, materials and procedures;

Report of cuttings transport flow-loop tests with foam, under several inclinations from vertical to horizontal;

Results of experimental tests with foam/aerated fluids in a full scale instrumented well for validation of the hydraulics software package and drilling operational procedures;

Integrated hydraulics software – vs. 2.0;

Drilling operational procedures manual – vs. 2.0.


computer programs will include technical background and user manual

Experiments (lab, flow-loops, full-scale)

test matrix for each kind of experiment to be discussed and approved by steering committee;

tests can be followed by representatives from any participant company.

4.3. Four-phase Separator

Report with complete design of a preliminary two-phase separator

Report of experiments with two-phase separator at Petrobras research well

Report with complete design of the four-phase separator

Report of experiments with the four-phase separator at Petrobras research well

4.4. Floating Rig Application

Report with analysis of potential problems and limitations

Offshore specific drilling operational procedures, including hydraulics aspects in the riser

Itemized circulation system definition and deployment

4.5. Wellbore Stability

SIGMA, a wellbore stability simulator for drilling and production, including user’s manual

5. Available Resources

5.1. Research Well Characteristics

Image19Well 9-PE-2-TQ-BA (Figure 1), located in Taquipe Field, onshore, Brazil, is a 1300 m well with 13 3/8″ casing. Inside it, a 7″ casing plays the role of the test well. Between both casings, several side lines are run with the purpose of allowing gas injection at bottom or around 770 m and, also, for simulation of offshore operations in floating platforms at 760 m of water depth, with submarine kill and choke lines.

The well is fully instrumented, permitting real time data collection of pressures and temperatures in all surface lines and at the bottom of the well. Besides of that, it is possible to acquire bottom-hole data on fluid density, down-hole total flow rate and gas fraction through an available logging unit. Finally, pressure and temperature memory sensors can be positioned at any desired position along the well (Figure 2).

5.2. Research Labs

All the necessary facilities, such as drilling fluids and cementing labs, including flow loops for hydraulics and cuttings transport experiments, core sample analysis and rock mechanics labs, are available for this project at Petrobras’ Research Center (CENPES), in Rio de Janeiro. A brief description of each one is given ahead:

Rock Mechanics Lab: carries out geo-mechanical tests in core samples for determination of static and dynamic elastic parameters, rock resistance, pore collapse with simultaneous measurement of permeability and determination of in-situ stresses with SR3D equipment.

Rock-fluid Interaction Lab: evaluates, with the help of physical simulators, the interaction between drilling and completion fluids with the formation rocks.

Image20Sample Preparation Lab: prepares core samples for geo-mechanical tests, formation damage evaluation and acidification tests; besides of that, it develops special procedures for non-conventional tests with shales and unconsolidated sands.

Drilling Fluids Lab: performs the appraisal of new additives and systems used in the preparation of drilling fluids

Rheology Lab: provides rheological characterization, including description of viscous-elastic properties in high pressure and/or high temperature for any fluid.

Acidification Lab: determines the ideal formulations for sand acidification, as a function of formation mineralogy, permeability and temperature.

Hydraulics Conductivity Lab: evaluates physical, chemical and mechanical properties of propant agents used in hydraulic fracturing jobs and sand containment.

Cement Lab: conducts tests for determination of physical properties of cement slurries, such as thickening time, fluid loss, compressive resistance and permeability; also, it performs tests for granular distribution definition of cement and solid additives.

Physical Simulation Lab: it executes experimental jobs in scaled down models for description of transport phenomena during drilling, cementing and completion operations. The most important physical simulators in this lab are the cuttings transport flow-loop, fluidization units, bottom-hole conditions (pressure and temperature) simulator, surface hydraulic simulator, simulator for gas migration evaluation in cemented annulus, flow-loop for kicks and multiphase flow analysis and thermo-hydraulic simulator (under construction).

6. Project Team

Edson Y. Nakagawa – Coordinator

M. Sc. in Petroleum Engineering from University of Ouro Preto, Brazil, 1986, and Ph. D. in Petroleum Engineering from Louisiana State University, USA

Working for Petrobras since 1980; joined Petrobras Research Center in 1982, being, nowadays, involved in Well Engineering and Technology projects, specially those related to Petrobras’ Deep Water Program


Petrobras Research Center – CENPES/DIPLOT/SETEP

Ilha do Fundão, quadra 7

Rio de Janeiro, RJ 21949-900 – Brazil

Tel: 55-21-598-6537 Fax: 55-21-598-6795

E-mail: edson@cenpes.petrobras.com.br

Antonio Luiz Serra de Souza – Reservoir Assessment Group

E-mail: aluiz@cenpes.petrobras.com.br

M. Sc. in Mechanical Engineering from Catholic University of Rio de Janeiro (PUC), Brazil, 1988 and Ph. D. in Petroleum Engineering from Stanford University, USA, 1987.

Head of Reservoir Simulation Group at PETROBRAS Research Center, 1989 to 1983.

André Leibsohn – Cuttings Transport Group

E-mail: aleibsohn@cenpes.petrobras.com.br

B. Sc. in Chemical Engineering from Federal University of Rio de Janeiro, Brazil, 1985, and M. Sc. in Petroleum Engineering from State University of Campinas, Brazil, 1990.

Works for PETROBRAS Research Center, in Wellbore Technology Sector, since 1986, leading projects on cuttings transport, drilling hydraulics and rheology. Presently, coordinates a project on cuttings transport and cementing in highly inclined extended reach, multilateral and “designer wells” for deepwater environments.

Carlos A. M. Saliba – Foam Rheology and Formation Damage Group

E-mail: saliba@cenpes.petrobras.com.br

M. Sc. in Physical Chemistry from Federal University of Rio de Janeiro, Brazil, 1985, and Ph. D. in Physical Chemistry from University of Paris 6, France, 1996.

Works for Petrobras since 1985, leading projects on formation damage, rheology, fluid loss additives and wettability.

Álvaro Maia – Wellbore Stability Simulation Group

E-mail: amaia@cenpes.petrobras.com.br

B. Sc. (1976), M. Sc. (1978) and D. Sc. (1984) in Civil Eng. from Federal University of Rio de Janeiro, Brazil

Head of the Numerical Modeling and Development of Methods Group at PETROBRAS Research Center

José Carlos de Souza Cunha – Offshore Implementation Group

E-mail: cunhajc@ep.petrobras.com.br

M. Sc. in Petroleum Engineering from University of Ouro Preto, Brazil, 1988, and Ph.D. in Petroleum Engineering from Tulsa University, USA, 1995.

Coordinator of Shallow Reservoirs Located in Deep and Ultra-deep Waters Project from Petrobras´ Deep Water Program

Fernando de Almeida França – Four-phase Separator Group

E-mail: ffranca@fem. unicamp.br

B. Sc. from University of Brasilia, Brazil, M. Sc. and Ph.D. from State University of Campinas (UNICAMP), Brazil;

Professor of Department of Energy at UNICAMP

Coordinates Multiphase Phenomena Lab at Mechanical Eng. Dept. at UNICAMP involved in multiphase flow projects and instrumentation for fluids and heat experiments

7. Costs and Funding

The total estimated cost for this project, along 17 months (Aug, 1997 – Dec, 1998 – schedule in Annex A), is US$ 900,000.00 distributed as:


Reservoir assessment: 72,000.00

Drilling: 738,000.00

Wellbore stability: 90,000.00

TOTAL: 900,000.00

PETROBRAS will provide the facilities and other resources needed for the experiments (research well and labs) and field tests. The numbers shown above include just the costs related to (1) estimated man-hours of Petrobras personnel; (2) materials and (3) sub-contractors.

It is expected that six companies would join this project. Therefore, the cost per participant would be US$ 150,000.00, to be paid according to the schedule proposed in the contract (Annex B).