Permeability Loss In Depletion Of Reservoirs
Authors: Marek Ben F. 
Published: December 2007

Laboratory hydrostatic stress measurements and petrophysical analysis were made on full core samples petrophysical analysis were made on full core samples from a Middle East reservoir. The samples yielded under hydrostatic stress by pore collapse. This pore collapse reduced the permeability. The yield strength increased initially on some of the samples, but at higher hydrostatic confining stress, it decreased until the point where it yielded under hydrostatic stress. The tests were run with the pores 100% saturated with simulated reservoir brine. pores 100% saturated with simulated reservoir brine. The pore pressure was maintained at atmospheric pressure and the temperature was maintained at room pressure and the temperature was maintained at room temperature.

Petrophysical analysis of the core samples prior to and after the stress measurements showed the material to be a poorly cemented, chalk-like limestone having a weak matrix strength. A majority of the samples showed permanent pore deformation and a significant decrease in permeability after they had been submitted to effective stresses in the range of 40.8 MPa (6,000 psig).

Introduction

Hydrocarbons have been found in chalk-like limestone reservoirs in the North Sea, the Middle East, the Gulf Coast and Midcontinent regions of the U.S., and the Scotian Shelf of Canada. The development of production operations and reservoir elevation of production operations and reservoir elevation of these types of reservoirs requires a knowledge of the effect of reservoir stress on the productivity of individual wells and the overall behavior of the reservoir energy contributed by the compressibility of the reservoir formation.

There are many correlations between reservoir stress and porosity and permeability available in the published literature. These correlations, however, often cannot be used to describe the effect of stress on high-porosity, weak frame strength limestone material as characterized by this Middle East reservoir. Laboratory tests showed that the formation does not behave as an elastic material under stresses slightly above the estimated initial stress, but tends to yield and permanently deform. This collapse of the pore structure cam decrease drastically the permeability, and, in turn, the productivity of the reservoir. This type of productivity of the reservoir. This type of permeability loss cannot be restored necessarily by permeability loss cannot be restored necessarily by acidizing, fracturing or other well stimulation techniques. Early detection of high permeability loss in zones of weak matrix strength is very important in planning production operations and pressure maintenance programs to obtain optimum pressure maintenance programs to obtain optimum production rates from these type reservoirs. production rates from these type reservoirs. The objective of this paper is to describe the possible stress effects on productivity due to possible stress effects on productivity due to pressure depletion and to suggest that this pressure depletion and to suggest that this phenomena be taken into consideration when planning phenomena be taken into consideration when planning production schedules for individual wells and the production schedules for individual wells and the field as a whole.

DESCRIPTION OF RESERVOIR SAMPLES TESTED

All the samples examined were limestones and most were poorly consolidated. The limestones consisted of various allochems (carbonate grains) cemented in a matrix, lime mud, and finely crystalline calcite. The allochems consist of intraclasts, pelloids, and fossils: foraminifera, echinoid spines and plates, and bivalve mollusk fragments. Many of the skeletal grains have undergone either micritization or recrystallization, or a combination of the two.

Forminifera are represented by several types, the most abundant and conspicuous of which are millolids and large orbitolinids and dicyclinids. Some of the dicyclinids have a tube-like structure and greatly resemble sponges. Some of the foraminifer types have been micritized thoroughly and thus are difficult and, in some instances, impossible to differentiate from intraclasts (fragments of lime mud).