Characterizing the Greater Burgan Field: Use of Geochemistry and Oil Fingerprinting

Summary This study reports reservoir geochemistry findings on the Greater Burgan field by a multidisciplinary, multiorganizational team. The major objectives were to determine if unique oil fingerprints could be identified for the major producing reservoirs and if oil fingerprinting could be used to identify wells with mixed production because of wellbore mechanical problems. Three potential reservoir geochemistry applications in the Burgan field are:evaluation of vertical and lateral hydrocarbon continuity,identification of production problems caused by leaky tubing strings or leaks behind casing, andallocation of production to individual zones in commingled wells. Results from this study show that while oils from the major reservoir units are different from each other, the differences are small. Furthermore, a number of wells were identified in which mixed oils were produced because of previous mechanical problems. Both transient pressure testing and distributed pressure measurements provided corroborative evidence of some of these findings. Other data show that Third Burgan oils are different in the Burgan and Magwa sectors, suggesting a lack of communication across the central graben fault complex. This finding supports the geologic model for the ongoing reservoir simulation studies. Success of the geochemistry project has spawned enlargement of the study in both size and scope. Introduction This paper describes the results from a joint project by Chevron- Texaco Overseas Petroleum, the Kuwait Oil Co. (KOC), and the Kuwait Inst. for Scientific Research (KISR). Approximately 50 oils were analyzed to assess the feasibility of applying reservoir geochemistry in the Burgan field. All analytical work was performed at KISR. In this study, we report on a subset of these oils that contain primarily single-zone production samples. Reservoir geochemistry involves the study of reservoir fluids (oil, gas, and water) to determine reservoir properties and to understand the filling history of the field. Many established methods for exploration geochemistry can be used for this purpose. Reservoir geochemistry differs from other reservoir characterization methods by dealing primarily with the detailed molecular properties of the fluids in the C1-C35+ region rather than the physical properties. Larter and Aplin1 offer a review of many of these methods. Geochemistry techniques have been used to help solve reservoir problems for many years. During this time, oil geochemistry has been applied to the following reservoir characterization and management problems:Evaluation of hydrocarbon continuity.Analysis of commingled oils for production allocation.Identification of wellbore mechanical problems.Evaluation of workovers.Production monitoring for enhanced oil recovery (EOR).Identification of reservoir fluid type from rock extracts.Characterization of reservoir bitumens and tar mats. Many different analytical techniques have been used in these reservoir geochemistry studies. One of the most widely used is gas chromatography (GC). When used for oil correlation, it is often referred to as oil fingerprinting. In most reservoirs, the oil composition represents a unique fingerprint of the oil that can be used for correlation purposes.2 This is an inexpensive method and can be very cost-effective when compared to many production-logging methods. Of course, we recommend verifying this technique with other methods before reducing these more costly measurements. A number of papers have documented the application of oil fingerprinting to Middle East oil fields.3–7 Based on these studies, we felt that there was a high probability of success in using reservoir geochemistry in Kuwait's Burgan field. Three applications were of specific importance. Reservoir Continuity. The Burgan field contains several major producing horizons: the Wara, Third Burgan (Upper, Middle, and Lower), and Fourth Burgan reservoirs. Each of these is further subdivided into several reservoir layers. Vertical compartmentalization of the field, both in geologic and production time frames, is possible. In addition, a number of faults have been mapped in the field, and these may act as lateral barriers to fluid flow. The most significant faulting occurs in the central graben fault complex that separates the Burgan and Magwa/Ahmadi sectors of the field. Oil fingerprinting, along with other oilfield data, will be used to evaluate vertical and lateral compartmentalization in the field. Tubing-String Leaks. In many older fields, the integrity of casing strings and cement bonding is often a problem. If multiple pay zones are present, oil may leak into or behind the casing string from zones other than the completion interval. Many wells in the Burgan field produce from two reservoirs. Some wells, for example, produce Wara oil up the annulus and Third Burgan oil up the tubing string. When fingerprints of the individual oil zones have been identified, wellhead samples of the two production streams can be analyzed to determine if a mechanical problem is present.2,8 Production Allocation. It has been shown that the relative proportions of individual oils in an oil mixture can be determined with GC.9,10 Using this method to analyze production streams provides a rapid means of production allocation and does not require that wells be taken off production. In the Burgan field, this method will be applied to evaluate the extent of oil mixing either in the wellbore, owing to mechanical problems, or in the reservoir because of crossflow from deeper, higher-pressure reservoirs. The Burgan Oil Field The Greater Burgan oil field lies within the Arabian basin in the state of Kuwait. General reviews of the geology and producing history of the field are described by Brennan11 and by Kirby et al.12 The field is subdivided into the Burgan, Magwa, and Ahmadi sectors, based on the presence of three structural domes. Fig. 1 shows that the northern Magwa and Ahmadi sectors are separated from the southern Burgan sector by a central graben fault complex.