Overview of Formation Damage
Reservoir Formation Damage
Formation damage is a generic terminology referring to the impairment of the permeability of petroleum bearing formations by various adverse processes. Formation damage is an undesirable operational and economic problem that can occur during the various phases of oil and gas recovery from subsurface reservoirs including production, drilling, hydraulic fracturing, and workover operations. As expressed by Amaefule et al. (1988) "Formation damage is an expensive headache to the oil and gas industry." Bennion (1999) described formation damage as: "The impairment of the invisible, by the inevitable and uncontrollable, resulting in an indeterminate reduction of the unquantifiable!" Formation damage assessment, control, and remediation are among the most important issues to be resolved for efficient exploitation of hydrocarbon reservoirs (Energy Highlights, 1990). Formation damage is caused by physico-chemical, chemical, biological, hydrodynamic, and thermal interactions of porous formation, particles, and fluids and mechanical deformation of formation under stress and fluid shear.
These processes are triggered during the drilling, production, workover, and hydraulic fracturing operations. Formation damage indicators include permeability impairment, skin damage, and decrease of well performance. As stated by Porter (1989), "Formation damage is not necessarily reversible" and "What gets into porous media does not necessarily come out." Porter (1989) called this phenomenon "the reverse funnel effect." Therefore, it is better to avoid formation damage than to try to restore it. A verified formation damage model and carefully planned laboratory and field tests can provide scientific guidance and help develop strategies to avoid or minimize formation damage. Properly designed experimental and analytical techniques, and the modeling and simulation approaches can help understanding, diagnosis, evaluation, prevention, remediation, and controlling of formation damage in oil and gas reservoirs.
The consequences of formation damage are the reduction of the oil and gas productivity of reservoirs and noneconomic operation. Therefore, it is essential to develop experimental and analytical methods for understanding and preventing and/or controlling formation damage in oil and gas bearing formations (Energy Highlights, 1990). The laboratory experiments are important steps in reaching understanding of the physical basis of formation damage phenomena. "From this experimental basis, realistic models which allow extrapolation outside the scaleable range may be constructed" (Energy Highlights, 1990). These efforts are necessary to develop and verify accurate mathematical models and computer simulators that can be used for predicting and determining strategies to avoid and/or mitigate formation damage in petroleum reservoirs (Civan, 1994).
Confidence in formation damage prediction using phenomenological models cannot be gained without field testing. Planning and designing field test procedures for verification of the mathematical models are important. Once a model has been validated, it can be used for accurate simulation of the reservoir formation damage. Current techniques for reservoir characterization by history matching do not consider the alteration of the characteristics of reservoir formation during petroleum production. In reality, formation characteristics vary and a formation damage model can help to incorporate this variation into the history matching process for accurate characterization of reservoir systems and, hence, an accurate prediction of future performance. Formation damage is an exciting, challenging, and evolving field of research. Eventually, the research efforts will lead to a better understanding and simulation tools that can be used for model-assisted analysis of rock, fluid, and particle interactions and the processes caused by rock deformation and scientific guidance for development of production strategies for formation damage control in petroleum reservoirs. In the past, numerous experimental and theoretical studies have been carried out for the purpose of understanding the factors and mechanisms that govern the phenomena involving formation damage. Although various results were obtained from these studies, a unified theory and approach still does not exist.
A formation damage model is a dynamic relationship expressing the fluid transport capability of porous medium undergoing various alteration processes. Modeling formation damage in petroleum reservoirs has been of continuing interest. Although many models have been proposed, these models do not have the general applicability. However, an examination of the various modeling approaches reveals that these models share a common ground and, therefore, a general model can be developed, from which these models can be derived. Although modeling based on well accepted theoretical analyses is desirable and accurate, macroscopic formation damage modeling often relies on some intuition and empiricism inferred by the insight gained from experimental studies.As J. Willard Gibbs stated in a practical manner: "The purpose of a theory is to find that viewpoint from which experimental observations appear to fit the pattern" (Duda, 1990).
The fundamental processes causing damage in petroleum bearing formations are:
4 thermal, and
Formation damage studies are carried out for:
1 understanding of these processes via laboratory and field testing,
2 development of mathematical models via the description of fundamental mechanisms and processes,
3 optimization for prevention and/or reduction of the damage potential of the reservoir formation, and
4 development of formation damage control strategies and remediation methods.
These tasks can be accomplished by means of a model assisted data analysis, case studies, and extrapolation and scaling to conditions beyond the limited test conditions. The formulation of the general purpose formation damage model is presented by describing the relevant phenomena on the macroscopic scale; i.e. by representative elementary porous media averaging.
Development of a numerical solution scheme for the highly non-linear phenomenological model and its modification and verification by means of experimental testing of a variety of cores from geological porous media are the challenges for formation damage research. As expressed by Porter 1989 and Mungan 1989, formation damage is not necessarily reversible. Thus, it is better to avoid formation damage than try to restore formation permeability using costly methods with uncertain successes in many cases. When a verified generalized formation damage model becomes available, it can be used to develop strategies to avoid or minimize formation damage.
Finally, it should be recognized that formation damage studies involve much interdisciplinary knowledge and expertise. An in-depth review of the various aspects of the processes leading to formation damage may require a large detailed presentation. Presentation of such encyclopedic information makes the learning of the most important information difficult and, therefore, it is beyond the scope of this book. Instead, a summary of the well proven, state-of-the-art knowledge by highlighting the important features, are presented in a concise manner for instructional purposes. The details can be found in the literature cited at the end of the chapters.
Common Formation Damage Problems, Factors, and Mechanisms
Barkman and Davidson 1972, Piot and Lietard 1987, and Amaefule et al. 1987, 1988 have described in detail the various problems encountered in the field, interfering with the oil and gas productivity. Amaefule et al. 1988 listed the conditions affecting the formation damage in four groups:
1 Type, morphology, and location of resident minerals;
2 In-situ and extraneous fluids composition;
3 In-situ temperature and stress conditions and properties of porous formation; and
4 Well, development and reservoir exploitation practices.
Amaefule et al. 1988 classified the various factors affecting formation damage as follows:
1 Invasion of foreign fluids, such as water and chemicals used for improved recovery, drilling mud invasion, and workover fluids;
2 Invasion of foreign particles and mobilization of indigenous particles, such as sand, mud fines, bacteria, and debris;
3 Operation conditions such as well flow rates and wellbore pressures and temperatures; and
4 Properties of the formation fluids and porous matrix.
Bennion 1999 delineates the common formation damage mechanisms in the order of significance. Bishop 1997 summarized the seven formation damage mechanisms described by Bennion and Thomas 1991, 1994 as following:
1. Fluid-fluid incompatibilities, for example, emulsions generated between invading oil based mud filtrate and formation water.
2. Rock-fluid incompatibilities, for example, contact of potentially swelling smectite clay or deflocculate kaolinite clay by nonequilibrium water-based fluids with the potential to severely reduce near wellbore permeability.
3. Solids invasion, for example, the invasion of weighting agents or drilled solids.
4. Phase trapping/blocking, for example, the invasion and entrapment of water-based fluids in the near wellbore region of a gas well.
5. Chemical adsorption/wettability alteration, for example, emulsifier adsorption changing the wettability and fluid flow characteristics of a formation.
6. Fines migration, for example, the internal movement of fine particulates within a rock's pore structure resulting in the bridging and plugging of pore throats.
7. Biological activity, for example, the introduction of bacterial agents into the formation during drilling and the subsequent generation of polysaccharide polymer slimes which reduce permeability.
Team for Understanding and Mitigation of Formation Damage
Amaefule et al. 1987, 1988 stated that formation damage studies require a cooperative effort between various professionals. These and their responsibilities are described in the following:
1 Geologist and geochemist on mineralogy and diagenesis and reservoir formation characterization and evaluation;
2 Chemist on inorganic/organic chemistry, physical chemistry, colloidal and interfacial sciences, and chemical kinetics; and
3 Chemical and petroleum engineers on transport phenomena in porous media, simulator development, interpretation of laboratory core tests, scaling from laboratory to field, interpretation of field tests, and development and implementation of strategies for formation damage control.
Amaefule, J. O., Ajufo, A., Peterson, E., & Durst, K., "Understanding Formation Damage Processes," SPE 16232 paper, Proceedings of the SPE Production Operations Symposium, Oklahoma City, Oklahoma, 1987.
Amaefule, J. O., Kersey, D. G., Norman, D. L., & Shannon, P. M., "Advances in Formation Damage Assessment and Control Strategies", CIM Paper No. 88-39-65, Proceedings of the 39th Annual Technical Meeting of Petroleum Society of CIM and Canadian Gas Processors Association, Calgary, Alberta, June 12-16, 1988, 16 p.
Barkman, J. H., & Davidson, D. H., "Measuring Water Quality and Predicting Well Impairment," Journal of Petroleum Petroleum Technology, Vol. 253, July 1972, pp. 865-873.
Bennion, D. B., Thomas, F. B., & Bennion, D. W., "Effective Laboratory Coreflood Tests to Evaluate and Minimize Formation Damage in Horizontal Wells," presented at the Third International Conference on Horizontal Well Technology, November 1991, Houston, Texas.
Bennion, D. B., & Thomas, F. B., "Underbalanced Drilling of Horizontal Wells: Does It Really Eliminate Formation Damage?," SPE 27352 paper, SPE Formation Damage Control Symposium, February 1994, Lafayette, Louisiana.
Bennion, D. F., Bietz, R. F, Thomas, F. B., & Cimolai, M. P., "Reductions in the Productivity of Oil & Gas Reservoirs due to Aqueous Phase Trapping," presented at the CIM 1993 Annual Technical Conference, May 1993, Calgary.
Bennion, B., "Formation Damage—The Impairment of the Invisible, by the Inevitable and Uncontrollable, Resulting in an Indeterminate Reduction of the Unquantifiable!" Journal of Canadian Petroleum Petroleum Technology, Vol, 38, No. 2, February 1999, pp. 11-17.
Bishop, S. R., "The Experimental Investigation of Formation Damage Due to the Induced Flocculation of Clays Within a Sandstone Pore Structure by a High Salinity Brine," SPE 38156 paper, presented at the 1997 SPE European Formation Damage Conference, The Hague, The Netherlands, June 2-3 1997, pp. 123-143.
Civan, F, Predictability of Formation Damage: An Assessment Study and Generalized Models, Final Report, U.S. DOE Contract No. DE-AC22- 90-BC14658, April 1994.
Civan, F., "A Multi-Purpose Formation Damage Model," SPE 31101 paper, Proceedings of the SPE Formation Damage Symposium, February 14-15, 1996, pp. 311-326, Lafayette, Louisiana.
Duda, J. L., "A Random Walk in Porous Media," Chemical Engineering Education Journal, Summer 1990, pp. 136-144.
Energy Highlights, "Formation Damage Control in Petroleum Reservoirs," article provided by F. Civan, The University of Oklahoma Energy Center, Vol. 1, No. 2, p. 5, Summer 1990.
Mungan, N., "Discussion of An Overview of Formation Damage," Journal of Petroleum Technology, Vol. 41, No. 11, Nov. 1989, p. 1224.
Piot, B. M., & Lietard, O. M., "Nature of Formation Damage in Reservoir Stimulation, in Economides," M. J. and Nolte, K. S. (eds.), Reservoir Stimulation, Schlumberger Education Services, Houston, Texas, 1987.
Porter, K. E., "An Overview of Formation Damage," JPT, Vol. 41, No. 8, 1989, pp. 780-786.