Modelling Carbonate Reservoir Porosity For Enhanced Recovery Schemes
Authors: Kent D.M. 
Published: March 2010

Most carbonate rock porosity classifications are generic and do not account for features of the pore systems that may affect fluid movement and withdrawal-important factors in the production of hydrocarbons from these rocks.

Mercury injection capillary pressure (MICP) analyses of carbonate reservoirs supply valuable information on the interaction between non-wetting fluids and reservoir porosity. Some 200 analyses of a variety of Mississippian carbonate reservoirs in southern Saskatchewan indicate that porosity can be simplified into three types; inter-crystal, cavity and mixed (inter-crystal and cavity). The categories can be recognized by the degree of uniformity of pore throat sizes in the reservoir rock; a high degree of uniformity indicates inter-crystal porosity and a wide variation in pore throat size is representative of cavity porosity.

Entry pressure is a second MICP factor that tells something about the reservoir rock. High entry pressures are commonly related to calcitic reservoir rocks with inter-crystal porosity: intermediate entry pressure indicates dolomite with inter-crystal porosity; and low pressures are representative of calcitic or dolomitic rocks with abundant large cavities (mesopores).

Pore system/fluid interaction can be shown to influence efficient withdrawal of hydrocarbons from a reservoir, thus productivity may vary markedly due to differences in porosity types over the lateral extent of a producing zone.

Introduction

During the past ten years, several graduate students in the Department of Geology, University of Regina, and the author have investigated several geological aspects of the Mississippian reservoir rocks in southeastern Saskatchewan. These include studies into the microfacies, diagenesis and nature of the pore systems in the Benson (Midale Beds)(1), Glen Ewen (Midale Beds)(2), Innes (Frobisher Beds)(3,4) and Hummingbird (Ratcliff Beds)(5) oilfields. In addition to these completed investigations, a study is at present being carried out on three Ratcliffe Beds producing areas: Flat Lake, Lake Alma, and Oungre.

Most of these studies have been generously supported by the Saskatchewan Department of Energy and Mines, in an effort to establish a readily available source of information on carbonate rock reservoirs for enhanced recovery schemes. The Hummingbird study was financed through a grant to Dr. L. W. Vigrass from Hudson's Bay Oil and Gas Co. Ltd.

The pore systems of the reservoirs in these fields have been studied using mercury injection capillary pressure analyses, to establish pore throat size distribution and a variety of fluid response characteristics, including entry pressure, ejection efficiency and unsaturated pore volume. The results of these analyses were enhanced by detailed examination of the reservoir rocks using scanning electron and light microscopy.

The variety of carbonate rock types constituting the reservoirs in the Mississippian fields studied have made it possible for the writer to establish a simplified classification of porosity for carbonate reservoir rocks. Because Mississippian rocks are used as the basis for this classification a brief review of their important characteristics is presented below.

Geologic Setting

Mississippian rocks in southern Saskatchewan were deposited on the northern flank of the Williston Basin in a marine seaway in which paleobathymetry exerted an influence on the distribution of facies.