Abstract

Production of Arabian super light crude oil from the Upper Permian Unayzah and Khuff Formations, in central Saudi Arabia, began in 1994. Water flooding was started at the same time to maintain pressure support to the reservoirs. Because many of the oil wells started to produce wet crude, identification of the source of water (i.e., injection vs formation water) was needed to design a successful water shut-off treatment and identify whether early breakthrough had occurred. Geochemical characterization of the injection and formation waters, using chemical and stable isotope methods, reveals that each has a distinctive geochemical fingerprint that can be used to identify breakthrough and calculate mixing relationships. So far, the composition of produced water is identical to that of the aquifer underlying the reservoirs, which clearly demonstrates that breakthrough of injection water has not occurred. However, the flood front will eventually migrate to oil wells where it will mix with formation water. A program is in place to monitor the chemical and isotopic composition of produced water from oil wells. Because oxygen isotope ratios (18O values) are very conservative tracers of mixing the time of breakthrough will be signaled by a shift of the 18O value towards that of the injection water. The amount of injection water in the wet crude can be quantified with stable isotopes, using the known end-member compositions. This allows determination of which ions behave conservatively or are removed during mixing, and prediction of potential scaling problems. The sulfur isotope ratio of dissolved sulfate in the produced water shows a wide range of 34S values, suggesting the presence of sulfate reducing bacteria in the reservoir.

Introduction

In 1989, light (43 to 52 API), low sulfur crude oil was discovered in Paleozoic sandstone reservoirs in central Saudi Arabia, south of Riyadh. To date, more than a dozen fields have been discovered in Central Arabia. The reservoirs are in the Upper Permian Unayzah Formation and the basal clastic member of the overlying Khuff Formation. Oil production started in August of 1994 from four of these fields, and water injection began shortly afterward to maintain bottom water drive. Groundwater from the Triassic Jilh acquifer is injected into the aquifer underlying the Unayzah and Khuff reservoirs. Many of the oil wells started producing wet crude shortly after oil production began.

Identifying the source of the water in wet crude (i.e., coning of the underlying aquifer or early breakthrough of injection water) is important in order to design a successful water shut-off treatment, identify injection water breakthrough, and determine breakthrough times. This provides valuable information for reservoir engineers to evaluate injection water sweep efficiency. Breakthrough can be identified by adding chemical or isotopic tracers to the injection water, but this can be expensive or have severe restrictions (i.e., radioactive tracers) and provides no information until the tracer actually reaches the well-bore. If there are distinct chemical and/or isotopic differences between the formation and injection waters, they can provide geochemical fingerprints for the different water sources. Because the stable isotope ratios of oxygen and hydrogen are very conservative tracers of mixing, they are especially useful to identify and quantify breakthrough. This allows determination of which ions behave conservatively or are removed during mixing, and prediction of potential scaling problems.

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