Evaluation of Reservoir and Hydraulic Fracture Properties in Geopressure Reservoirs

Abstract

Field production performance data and multiple pressure transient tests over a period of time for oil and gas wells in geopressured reservoirs have been found to often exhibit marked changes in reservoir effective permeability over the producing life of the wells. Similarly, the use of quantitative fractured well diagnostics to evaluate the production performance of hydraulically fractured wells have clearly shown that effective fracture half-length and conductivity can be dramatically reduced over the producing life of the wells. This paper reports the results of the research, development, and application of production performance analysis models to directly measure the changes in the reservoir effective permeability and fracture effective conductivity of hydraulically fractured wells completed in reservoirs that are subject to stress-dependent variation of the intrinsic formation and fracture properties. The interpretation model development is documented in the Appendices of this paper. Field examples are presented which demonstrate the application of the interpretation models to both unfractured and hydraulically fractured well production performance analyses in which stress-dependence of the reservoir and fracture properties are clearly exhibited. Introduction A common assumption that is made in pressure transient test interpretation analysis or production performance modeling efforts is that the porous media that constitutes a hydrocarbon-bearing reservoir is rigid and non-deformable. While this assumption is reasonably valid for many commonly-occurring petroleum reservoir engineering problems related to fluid flow in relatively well-consolidated formations, there are many instances (particularly pertaining to geopressured reservoirs) when this assumption can not be validly made. It is often found that when dealing with geopressured reservoirs that even reasonably good estimates of the "average" formation properties are applicable over only a limited part of the production history of the reservoir. Some of the earliest references to the fact that subterranean reservoirs do not always behave as rigid and non-deformable bodies of porous media may be found in the groundwater hydrology literature1–3. Rowan and Clegg4 have provided a review of the fundamental equations governing fluid flow in porous media. An excellent numerical study of the effects of stress-dependent formation properties was reported by Raghavan et al 5. That study also first reported the use and development of linearization techniques for modeling fluid flow in stress-sensitive porous media which laid the groundwork for the work of Samaniego et al 6 and Samaniego and Cinco-ley7 in the development of pressure transient test interpretation analyses for reservoirs with stress-dependent permeability. The method of pressure transient test analysis estimation of stress-dependent permeability developed by Samaniego and Cinco-Ley7 also utilized a technique of estimating the sandface flowing pressure-dependent effective permeability of a reservoir in a manner analogous to that derived by Al-Khalifah et al 8. The Raghavan et al5, Samaniego et al6, and Samaniego and Cinco-ley7 studies provided synthetic numerical examples of the stress-dependent permeability effects on the pressure transient behavior of a well. Garg 9, Ostensen10, Pedrosa 11, Vairogs et al12, Vairogs and Rhoades13, and Thomas and Ward14 have also reported the results of investigations of stress-dependent effects on formation permeability.

Production Performance Design Criteria for Hydraulic Fractures

“…Obviously, this method can provide a significant fracture conductivity (k f b f ) advantage, provided that the fracture remains open! Unfortunately, during production this is generally not the case, especially in deep, tight formations or geopressured reservoirs 10 . As pressure changes, those areas of the fracture without proppant will tend to close and heal back due to increasing closure stress, reducing the effective fracture halflength (X f,eff ) and fracture conductivity (k f b f ), resulting in a corresponding loss in production.…”