Heterogeneity and Anisotropy Factors in Sidewall Core Selection, Acquisition, and Analysis
The need to validate and integrate assorted petrophysical logs with core measurements is key in defining formation properties, rock composition, diagenetic effects, clay typing, rock mechanical features, and reservoir geology and sedimentology. The objective of this integration is to understand the reservoir and maximize its productivity.
There is an array of applications for core analysis such as enhancing hydrocarbon recovery strategies, improving exploration campaigns, defining field development plans, refining reservoir description, and assembling key data for ultimate recovery. Conventional coring is an intrinsic part of all these elements owing to the value it brings, as there is no other data comparable to the actual physical rock that conveys irrefutable and irreplaceable evidence of the subsurface. Rotary sidewall coring (SWC) offers a time-effective and flexible alternative, in terms of coring intervals. Both methodologies can be complementary providing a comprehensive assessment of the subsurface.
This paper evaluates the impact that the SWC selection process has on operational efficiency, and also on the measurements of rock properties, that are directionally dependent, such as permeability, stress related, acoustic and electrically bound anisotropy. The program compilation must consider the analysis requirements, which include depth control and recovery qualification linked to the borehole conditions (washouts, breakouts, drilling-induced fractures, and invasion), expected rock mechanical properties, formation heterogeneities (i.e., laminations, natural fractures) and the reservoir quality.
The extent of formation anisotropy, heterogeneity and possible implications on the core analysis, should be assessed prior to engaging in the core selection and setting the recovery criteria. It is critical for appropriate core sampling to address reservoir potential; for instance, in conventional reservoirs (porous sands or carbonates), core testing is designed to predict the compaction behavior, propensity for sand production, and critical stress states leading to wellbore instability. In addition, laminated sedimentary rocks and other formation types, exhibit preferential directions of deformation and failure. In this latter case, the inclusion of laminae into SWC samples influences the outcome of laboratory tests in the case of measuring such properties as permeability, because an anisotropic element is present in the samples prior to testing.
Thus, planning of SWC sample point selection is crucial and the recovery can be optimized if these criteria are cohesively integrated and well defined, accounting for all the risks involved including the potential effects of their combination and interaction on resulting recovery of the samples, and how well they fit the purpose of the sampling objectives. The paper provides a guideline for a coring program compilation to enhance operational efficiency and quality of the final analysis results.