Formation Damage Laboratory Testing is routinely performed to select optimum drilling, completion and other fluids prior to their deployment in wells. Major decisions which impact the productivity or injectivity of wells and fields are often based on laboratory tests. This paper aims to resolve some of the arguments and diversity in the procedures for laboratory formation damage tests and present clear evidence to support opinions expressed on the merits of particular tests. The paper is deliberately non-reservoir specific and will formalise in one document evidence, opinions and suggested test improvements.
Over the past 18 years various laboratory projects have been designed and conducted by the authors to improve well productivity and injectivity by understanding and mitigating formation damage. One of the most important factors in design of laboratory testing for formation damage is to decide if the test objective is to compare fluids procedures or to attempt to simulate real well behaviour or both. The majority of formation damage testing referred to in this document should be viewed as a combination of simulation and comparative testing. Another critical factor in testing is that all tests are conducted at reservoir conditions in suitable equipment. All reservoir conditions wetted parts must be composed of corrosion resistant materials such as Hastelloy and Teflon. Testing in inferior quality equipment will be prone to artefacts and will generate erroneous results.
Samples for testing must be carefully selected and prepared in order to obtain results that are relevant for the reservoir under investigation. Laboratory testing, if performed carefully and with due consideration to sample selection and test design, can accurately measure potential reduction in permeability. In order to use this information and enhance productivity it is often necessary to determine the precise cause or causes of Formation Damage. Identifying damage and interpreting the results of advanced core flood tests usually requires a combination of engineering and geological skills. Geological techniques such as Scanning Electron Microscopy (SEM) and Thin Section analysis can be used to identify solid damaging mechanisms such as solids invasion, residual mud cakes, precipitates, fines migration or bacterial growth. Cryogenic SEM involves preparing part of the sample after the flood test down to liquid nitrogen temperature and analysis of this frozen specimen. This technique allows the identification of fluid damaging mechanisms such as wettability alteration, water blocks and micro-emulsion formation. By combining engineering and geological skills, it is not only possible to examine and simulate Formation Damage problems but diagnose those problems and help to prescribe the best treatment or the least damaging procedure.
One of the most important influences on Formation Damage is the geology of the reservoir. In order to apply the data produced from a laboratory test it is essential to understand the relevance of the rock material tested. Samples for laboratory testing must be selected to represent the major production or injection intervals. If there are significant differences in permeability or clay type, for example, these differences must be adequately represented in the material for testing. All available geological and relevant production data should be used in order to select the most suitable core material. Samples preserved with reservoir fluids in the pore space are preferred but old, dried out core may be restored to very close to the reservoir conditions using careful handling techniques. Special cleaning and drying procedures should be employed to ensure that samples selected are prepared without altering delicate structures in the pore space.
The laboratory tests are designed to incorporate all pressure changes in the near wellbore area. For example, if the well under investigation is to be returned to production by gradually decreasing the pressure in the wellbore (drawdown), then this step should be incorporated in the laboratory test.