Abstract

Laboratory determined salinities of water extracted quite different pore systems in terms of microfrom oil base cores recovered from the Elmworth porosity and bound water. area showed variations from as low as 6,000 ppm to over 60,000 ppm (NaCl eq.). It was observed that low salinities were associated with reservoirzones characterized by high cation exchange capacities and substantial feldspar content. By contrast, clean quartzose sandstones yielded relatively high salinities. Thin section and Scanning Electron ~Microscope examinations reveal that diagenetic processes affecting the feldspathic sandstones have produced significant quantities of secondary clays, especially chlorite, kaolinite and subsequent illite. As a result, the lithologic differences have been enhanced, leading to quite different pore systems in terms of microporosity and bound water.

Application of a Dual Water type model of shaly sands in conjuction with laboratory determined water saturation, porosity and cation exchangecapacity allowed estimation of free water salinity. This salinity was found to be in reasonable agreement with the salinity of produced waters.

Log derived water saturations are generally in good agreement with laboratory derived values.

Introduction

Formation water resistivity, Rw, is required for the determination of in-situ water saturation from resistivity logs. It is generally estimated from salinities of produced waters or from aconsideration of log responses in water saturated zones. Alternatively, Rw may be derived from salinities of waters extracted from oil base cores. However, because such waters are likely to be d mixture of "free" (or "formation") water and fresher "clay bound" water, 2, 3 measured salinities must be corrected before being employed in log analysis calculations.

An accurate knowledge of in situ water saturation is necessary to calculate hydrocarbon reserves of a reservoir, particularly for low permeability sands which are at the limit of economic producibi1ity.

We have demonstrated that the laboratory determined salinity of the connate water of reservoir rods is the mean water salinity of the waterbound to the clay surface and that of free water. Depending upon the degree of shaliness of the rock, the laboratory determined salinities were generally found to be 1ower than the actual salinity of the produced formation water.

Twenty three samples were selected from cores cut in oi1 base mud in the Elmworth Area of the so called "Deep Basin" of Western Canada. Porosity, cation exchange capacity (CEC), water saturation and water salinity were measured on each sample.

Measured water salinities were then corrected for dilution by clay water, using a "Dual Water" shaly sand model,2, 3 and compared with produced watersalinities.

EXPERIMENTAL PROCEDURES

Micro Extraction Salinity Determination Technique

We have developed in our laboratory a technique or the determination of the volume of connate water and its salinity for small amounts of rock samples. 5–10g of the rock was crushed and thoroughly ground in dehydrated methanol in an inert atmosphere of nitrogen gas. The water andsalt in the rock was extracted by methanol and a known volume of the solution was made.

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