The objective of this paper is to present the comparative results of comprehensive analysis of horizontal well productivity and completion performance with vertical wells drilled and completed within same time window in the Mauddud reservoir in the Bahrain Oil Field. The study also focuses on performance evaluation of horizontal wells drilled in different areas of the field. Key reservoir risks and uncertainties associated with horizontal wells are identified, and contingency and mitigation plans are devised to address them. Besides controlling gas production, the benefits of using cemented horizontal wells over vertical wells are highlighted based on performance of recently completed workovers and economic evaluation. Reservoir and well performance are analyzed using a variety of analytical techniques such as well productivity index (PI), productivity improvement factor (PIF), normalized productivity improvement factor (PIFn), well productivity coefficient (Cwp), in conjunction with a statistical distribution function to reflect the average and most likely values. In addition, average oil/gas/water production, cumulative production, reserves, and estimated ultimate recovery (EUR) are compared for both vertical and horizontal wells using decline curve analysis. Furthermore, economics are evaluated for tight spacing drilling with vertical wells, as well as horizontal cemented wells, to optimize future development of Mauddud reservoir. Based on the evaluation, it is inferred that the average horizontal well outperforms a vertical well in terms of production rate, PI, PIF, reserves, and EUR in the field except in waterflood areas. Based on average cumulative oil, reserves and EUR, and well productivity coefficient, overall performance of horizontal wells are better in the GI area in comparison their counterparts in the North/South areas of the Mauddud reservoir, where the dominant mechanism is strong water drive. High gas and water production in horizontal wells are attributed to open-hole completions of the wells and the possibility of poor cementing. A trial has been completed recently in a few horizontal wells using cased-hole cemented completion with selected perforations, resulting in improved oil rates and the drastic reduction of gas to oil ratio. Furthermore, two new cased-hole cemented horizontal wells are planned in 2021 as a trial. A detailed cost-benefit analysis using a net present value concept is performed, leading to a rethink of future development strategies with a mix of both vertical as well as horizontal wells in the GI area. Using the dimensionless correlations and distribution functions, the productivity and PIF of new horizontal wells to be drilled in any area can be predicted during early prognosis given the values of average reservoir permeability, well length, and fluid properties. This study can be used as a benchmark for the development of a thin oil column with a large and expanding gas cap under crestal gas injection using both vertical and horizontal wells.
Extended Abstract The Ahmadi formation of Bahrain field is of Middle Cretaceous age. It is predominantly a shale lying immediately below the Magwa member of the Rumaila formation and contains two limestone units in the Bahrain field, which are referred to as Aa and Ab members. Limestone Aa and Ab are present with practically uniform thickness over the entire Bahrain field area, as a blanket like deposition. The Ahmadi Reservoir in the Bahrain Field has been producing since 1933. Ahmadi consists of two main limestone units, AA and AB, separated by a 40-45 ft shale member. The AA reservoir is typically 3 to 4 feet thick, while the AB is divided into three separate units: AB1, AB2, and AB3. AB1 and AB3 are fairly clean limestone units, with a cumulative net reservoir of 12 to 14 ft. AB2 is about 4 to 5 feet thick, and characterized as a non-reservoir. The matrix permeability ranges from 1 to 2 mD. The main focus of the primary development plan was established by infill horizontal open hole lateral section, targeting the AB3 zone. However, poor matrix permeability and the irregularly spaced natural fracture network of the AB3 zone can hinder the primary development strategy and well production. Therefore, acid fracturing with 8 stages (60 ft each) in the open hole lateral section was implemented to improve well performance. A study was initiated in 2019 to improve the acid fracturing technique in the AB3 open hole lateral section (horizontal wells) by increasing the number of stages from 8 stages (60 ft each) to 18 stages (60 ft each). The wells targeted were located in areas with low reservoir quality and low fracture networks in order to induce artificial fractures, thereby improving well performance.
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