Forensic Petrophysics in Formation-Damage Evaluation

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 143988, ’The Role of Forensic Petrophysics in Formation- Damage Evaluation,’ by C. McPhee, SPE, M. Byrne, SPE, and G. Daniels, SPE, Senergy (GB) Limited, prepared for the 2011 SPE European Formation Damage Conference, Noordwijk, the Netherlands, 7-10 June. The paper has not been peer reviewed. Petrophysical analysis has a strategic role in evaluating well productivity and, ultimately, hydrocarbons in place. However, use in formation-damage assessment and evaluation has been overlooked or applied inappropriately. Overreliance on often ambiguous well-test data and a lack of understanding of the formation’s static and dynamic petrophysical properties have produced misleading conclusions on formation damage. Two field examples illustrate benefits of a forensic re-evaluation of log, core, and test data in both the recognition and rejection of formation damage in low-permeability oil and gas/condensate reservoirs in which data constraints precluded direct laboratory or field testing to quantify damage potential. Introduction Formation damage can be defined as any reduction in the near-wellbore permeability resulting from drilling, completion, production, injection, attempted stimulation, or any well intervention. Historically, attention has focused more on microscopic formation-damage mechanisms and macroscopic production-enhancement technology. Therefore, formation damage and its effect on well productivity often are overlooked such that potentially economic and prolific fields have been condemned on the basis of poor well productivity rather than poor permeability or connectivity. It is important to understand whether poor well productivity is a result of formation damage. Arguably, it is even more important to know if it is not. Forensic petrophysics involves d-tailed and integrated audit and analysis of log- and core-derived static and dynamic formation behavior, and provides a powerful tool for verifying well- and production-test results, particularly in the context of identifying and diagnosing formation damage. Two field cases are detailed in the full-length paper and demonstrate methods and techniques used in forensic-petrophysics solutions to recognize and reject formation-damage potential. Both examples are characterized by inherent uncertainties in the original formation-evaluation interpretation in data-constrained environments. Example 1: Offshore Oil Field The field was discovered in 1990. The sandstone reservoir section was drilled with potassium chloride/polymer water-based mud (WBM) and was cored over an interval of approximately 650 ft. Both routine-core-analysis (RCA) and special-core-analysis-laboratory (SCAL) tests were performed. Air permeability Ka ranged from 0.1 to 1,000 md and averaged 25 md. Endpoint oil permeability Ko measurements were made at representative stress and irreducible-water saturation Swir as part of the RCA (fresh-state) and SCAL (water/oil relative permeability) test programs. Petrographic tests indicated total clay content ranging from 1 to 11%—kaolinite predominates, although chamosite, corrensite (chlorite/smectite), and illite are present.