Introduction

Many carbonate reservoirs provide a major challenge for prediction of pay and of economic recovery of oil or gas. If a reservoir is comprised partially or totally of microporous flow units above the transition zone, those flow units frequently have a high indicated porosity on logs, a low calculated water saturation and strong gas or oil shows while drilling, yet yield uneconomic performance. Other carbonate reservoirs that are also microporous, however, yield significant volumes of oil or gas.

Identification and proper appraisal of hydrocarbon saturated microporous carbonate reservoirs is essential to minimize unnecessary costs for well testing and completion, and to avoid leaving bypassed pay. Case studies of various systems of microporous carbonate reservoirs help explain the differences in productivity and possible remediation methodology.

Theory

Before a case study can be evaluated, the characteristics of microporous rocks need to be understood. Each of these rock systems, when analyzed using mercury injection capillary pressure testing (figure 1), yield profiles similar in shape to the one shown, and plot on or above that profile. From this profile, it is evident that significantly great injection pressures are required for entry through the largest pore throats. Once adequate pressure is achieved to overcome pore throat entry, slight pressure increases will fill most of the additional pore volume through ports only slightly smaller than at entry pressure. This flat profile reflects the excellent sorting of the pore throat distribution. At pressures between 300-500 psi., microporous flow units achieve minimal water saturation values of 15% to 30%, in water wet reservoirs, with no free water.

These rocks also have common attributes such as low ratios of permeability/porosity, mean pore throat sizes less than 0.5 microns and pore volume greater than that expected for low flow layers.

Application

Unless a microporous flow unit is enhanced by natural fractures, is driven by rock compression (pore volume reduction as fluid is removed) or is in contact with macro porous flow units, low rates of gas flow or virtually no oil flow will result. Flow through the pore throats is also governed by the relative permeability to the hydrocarbon at the water saturation for the flow unit. Artificial stimulation by acid fracturing or hydraulic fracturing increases well bore surface area, significantly increasing short term flow, followed by rapid decline as flow is dominated by matrix flow away from the fracture face.

Five different productive, microporous reservoirs are documented in this paper. The methodology for identification and optimal completion techniques are included.

1. Danian Ekofisk-limestone

2. Austin Chalk-limestone

3. Lisburn field-carbonate mudstone

4. Weyburn field-sucrosic dolomite

5. Gilmer Field-impure carbonate mudstone

Conclusions

Early identification of the possible existence of micro-porous carbonate in an exploration or development scenario is most important. Use of well site drilling data, logs, production testing and laboratory tests separates microporous flow units from more “traditional” types of carbonate reservoirs.