Application of Variable Formation Compressibility for Improved Reservoir Analysis

Abstract

Formation compressibility has long been recognized as an important factor influencing production behavior from overpressured oil and gas reservoirs. However, formation compressibility data are not routinely collected and the use of formation compressibility in reservoir analysis and simulation is often oversimplified. This paper discusses more accurate methods to determine formation compressibility and introduces a new method for analyzing overpressured oil and gas reservoirs which utilizes the variability of formation compressibility with declining reservoir pressure. The newly developed method departs from earlier proposed methods in the use of variable rather than~formation compressibility by employing a "pore volume formation volume factor", Bt, that properly integrates pore volume compressibility effects over the full pressure range of investigation. Using the new concept of Bt, the material balance equation (MBE) can be modified to include the effects of pressure dependent formation compressibility.We find that the formation compressibility in highly overpressured unconsolidated reservoirs can be the same order of magnitude as gas compressibility and significantly higher than oil compressibility. In some types of reservoirs, an order of magnitude change in formation compressibility can occur during drawdown. We show that in many overpressured and/or unconsolidated reservoirs, proper integration of accurate formation compressibilities is important for reserve estimates, determination of drive energies, and overall reservoir development plans. For example, we find that the use of compressibility values in the MBE which are significantly lower than those which exist in the reservoir could suggest a strong water drive where one does not exist.

A Quadratic Cumulative Production Model for the Material Balance of an Abnormally Pressured Gas Reservoir

“…We will note that this model appears to function very well for the cases that Bourgoyne, et al Our remaining discussions of material balance models will simply address other models that have been proposed in recent times, and give a brief perspective on the utility of such models. Yale, et al 6 proposed a modified formulation of a material balance that is analogous in approach to that of Fetkovich, et al 1 In reference 7, Ambastha proposes and validates a "type curve" concept for a p D (dimensionless p/z function) versus G pD (dimensionless G p function) for the case of an abnormally pressured gas reservoir.…”

“…Each of these 6 (six) plotting functions (PF 1 -PF 6 ) is used to provide unique insight into the character of the basis function (Eq. 3.8) -we will present an illustrative example of these functions next.…”

Application of rate‐and‐state friction laws to creep compaction of unconsolidated sand under hydrostatic loading conditions

“…[2] Given that the deformation of most unconsolidated sands has been observed to depend on both state (contact area or porosity) and deformation rate, it is necessary to include these effects when constructing an appropriate constitutive law [Yale et al, 1993;Ostermeier, 1995;Chang et al, 1997;Hagin and Zoback, 2004]. A natural choice for such a law is the rateand-state variable friction law, since it already contains the necessary terms and was empirically derived from laboratory observations in a concise mathematical form [e.g., Dieterich, 1978Dieterich, , 1979 and has a physical basis in thermally activated creep at high-stress asperity contacts [Nakatani, 2001;Rice et al, 2001;Beeler, 2004;Nakatani and Scholz, 2004].…”