Identification and Exploitation of a High-Producing Field Extension With Integrated Reservoir Analysis

Summary The Wafra field is located in the partitioned neutral zone between Kuwait and Saudi Arabia. The field produces oil from the Ratawi oolite reservoir, which has been under production since 1956. Barriers to fluid movement have severely restricted aquifer support to the overlying carbonate grainstone reservoir, leading to production-induced pressure depletion and low recovery rates. Creative integration of three-dimensional seismic aspects, well log stratigraphy, and engineering analysis revealed an unexploited reservoir extension that is more open to aquifer pressure support. Wells drilled along this extension are expected to yield higher initial production rates and longer sustained production. The model was used to drill two successful step-out wells that have increased field production by over 12,000 BOPD. Eight of ten additional wells have now been drilled as a follow-up to this success. This paper reviews the case history with a focus on the multidisciplinary integration that led to opportunity identification and exploitation. Introduction The Wafra field, jointly operated by Saudi Arabian Texaco and the Kuwait Oil Company, is located in the partitioned neutral zone (PNZ) between Kuwait and Saudi Arabia (Fig. 1). The field has produced from the Lower Cretaceous Ratawi reservoir since 1956. Liquid withdrawal over the years has depleted reservoir pressures in some parts of the field, leading to a decline in production. Based on prior reservoir characterization and simulation studies in 1995,1 a peripheral water injection and an extension development program have been undertaken in order to arrest the decline and increase the production by over 40,000 BOPD.2 In 1996, a 104 sq mile three-dimensional (3D) seismic survey was acquired to help design and implement these programs. Structural and Stratigraphic Framework The Wafra field is a large anticline approximately 6 by 10 miles in dimension (Fig. 2). The field is composed of a main NW-SE trending structural feature called the Main area, and a lower amplitude extension area east of the Main area called East Wafra. The Ratawi oolite reservoir is found at a depth of 6,135 ft subsea at the structural crest and had an original oil-water contact at about 6,520 ft subsea. Most of the structuring occurred in Middle Cretaceous time as sediment draped over deep-seated horst blocks. Oil migration and accumulation are thought to have occurred primarily in Early Eocene time. The Ratawi formation consists of a marine transgressive sequence of carbonate rocks deposited during Early Cretaceous time. The formation is composed of three distinct intervals: the lower-most Ratawi oolite reservoir, and the overlying Ratawi Limestone and Ratawi Shale cap rocks (Fig. 3). The Ratawi oolite reservoir was formed by a prograding carbonate sand shoaling sequence deposited on a low-angle carbonate ramp or detached platform. The commercially productive reservoir interval is composed primarily of porous grainstones and packstones. Less porous packstone, mudstone, and wackestone facies resulted from a more-restricted lagoonal environment in the central part of the field, and deeper marine shelf facies on the platform boundaries. Stratigraphic analysis of well log data provides an understanding of the depositional framework and serves as a basis for modeling facies distribution within the reservoir. Fig. 4a is a well log cross section traversing the Main area and East Wafra along the path A-B in Fig. 2. The gamma ray (GR) log curves are flattened on the base of the Ratawi limestone (cap rock) and span the interval of the Ratawi oolite reservoir. The GR curves indicate a remarkable character similarity from well to well that is almost exclusively related to the presence of uranium minerals.1 This determination is supported by x-ray analysis of core data that found an absence of clay. Additional evidence is found in comparisons of the GR (uranium, potassium and thorium) with the computed GR (potassium and thorium) from spectrometry gamma ray logs. The computed GR data show a largely diminished log character, implying that the GR log character is largely a function of uranium content. Hence, the correlative nature of the GR curves indicates that the uranium was present at the time of deposition—probably due to regional-scale climatic or environmental influences such as atmospheric fall-out from volcanic activity. This explains the consistent levels of uranium, independent of lithology and porosity, and allows detailed chronostratigraphic correlations to be made. Fig. 4b is an east-west stratigraphic cross section through the reservoir along the same path as in Fig. 4a, showing porosity logs with GR depositional time lines superimposed. The thick solid lines mark lithostratigraphic boundaries between an interval consisting primarily of porous grainstone, which for purposes of this paper will be referred to as the "upper reservoir," a tight interval of predominantly mudstone and packstone, referred to as the "Basal barrier," and a porous grainstone interval called the "lower reservoir." Almost all of the Ratawi oil production is from the upper reservoir grainstones. The chronostratigraphic facies heterogeneity evident in Fig. 4b owes its origin to a transgressive sequence of prograding grain shoals deposited in relatively shallow water.3 During the early stages of transgression, as the shoals prograded over the Wafra paleo-high, muds and finer grain carbonates were deposited in intershoal lagoons. As the sea level rose, carbonate sediment productivity and accumulation surpassed the rise in sea level, resulting in an overall shallowing with time. Evidence of this can be seen in the general coarsening upward character of the porosity logs. With progressively shallower water depths and associated higher depositional energy, the grain shoals became spatially more extensive while the lagoonal areas retreated, ending in fairly expansive grain shoals in the later stages of reservoir development. At the end of Ratawi oolite time, a rapid increase in relative sea level drowned the shoaling sequence, and deposited the deeper marine Ratawi limestone and shale members.