Compaction Effects on Porosity and Permeability: Deepwater Gulf of Mexico Turbidite
Authors: Ostermeier R.M. 
Published: June 2014

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized to be experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering.

Introduction

Reservoir compaction continues to be a significant concern for the oil and gas industry. This is particularly true in the deepwater environment, where very high development costs make it essential to understand the long-term behavior of producing reservoirs. Furthermore, many deepwater reservoirs are prone to compaction. For example, typical deepwater Gulf of Mexico (GOM) reservoirs are unconsolidated to slightly consolidated Miocene-, Pliocene-, and Pleistocene-age turbidite sands. Such sands have potential for considerable compaction, especially if significant depletion occurs during the productive life of the reservoir. At discovery, most deepwater GOM prospects are highly geo-pressured and at the highest effective stress in their geologic life. In many cases, aquifer support is limited and water injection for pressure maintenance may be problematic. In these situations, economic development may require large depletions on the order of several thousand psi.

Compaction raises at least three major issues. One concerns seafloor subsidence and its impact on structure design and performance. A recent well-known example of this is Ekofisk1 in the North Sea.

Another issue is casing integrity. A compacting sand several tens of feet thick can impose severe and damaging stresses on production casing. Casing failure resulting from compaction has occurred in many fields throughout the world,2,3 but the high well cost in deep water magnifies the impact. The problem is compounded further in a stacked-pay situation, where pass-through wells may be seriously at risk.

There is also the impact of compaction on reservoir productivity. Compaction can aid dramatically in production by squeezing oil from the rock into the wellbore. However, compaction can also impair permeability and reduce production. Understanding the interplay of these effects for various production scenarios is essential for optimum reservoir management.

This paper presents a brief review of some of the work that has been done to understand better the effects of compaction on porosity and permeability for deepwater GOM turbidites. Important points to note are the following.Laboratory measurements and field observations show that pore-volume (PV) compressibility of deepwater GOM turbidite sands exhibits large variations in both magnitude and stress dependence. The key factors controlling these variations include geological age, sand morphology, and sand composition. Two sand "end members" can be identified.4,5 One exhibits relatively low initial compressibility that continues to decrease monotonically with increasing effective stress. This sand typically is older geologically; is characterized by predominantly long grain contacts; and has little, if any, load-bearing ductile material.The other end member may exhibit somewhat higher initial compressibility. Softening continues with increasing effective stress to some maximum value of compressibility, which may be several times larger than the initial value of either end member. Ultimately, softening ceases and the sand begins to harden as stress is increased further. This sand typically is younger geologically, has predominantly point-grain contacts, and has a significant amount of load-bearing ductile material.Compaction is known to affect permeability strongly. The precise nature of this effect, however, previously was not entirely clear, primarily because of the use of single-phase-brine permeability measurements and concomitant concerns related to fines migration.6 Recently, oil permeability measurements at initial water saturation, Swi, were made to eliminate any spurious effects caused by fines migration and to assess unambiguously the effects of stress on permeability.4,5,7 As expected, compaction effects on permeability are indeed quite significant. On a relative basis, compaction reduces permeability four to five times more than it reduces porosity.

The precision and accuracy of these oil-permeability data provided the impetus for developing a new permeability model7 based on simple scaling arguments. The model is in good agreement with the measurements and provides a reasonably reliable means of estimating permeability given porosity, water saturation, and grain-size distribution.

Core Sampling, Characterization, and Preparation

Core sampling, characterization, and preparation procedures are important concerns in understanding rock (i.e., sand) behavior. Therefore, it is important to review these issues, especially as they pertain to compaction testing.