Reliable and Cost Effective Reservoir Characterization of Thin, Layered and Heterogeneous Reservoirs: Case Studies

Abstract

In this era of cutting-edge technology where stochastic and geostatistical modelling is the order of the day, conventional layer-cake modelling for highly heterogeneous reservoirs in two fields have yielded very good results. The performance predictions of reservoir simulation studies based on the conventional geological model of these fields are in close conformance with the actual production behaviour thus substantiating the efficacy of such type of modelling. The results of these studies recommended that additional wells be drilled for augmenting production and recovery. Mapping of advancing edge-water front from simulation based on the 3D seismic derived geological model helped in optimal positioning of the development well locations. Although the role and accuracy of geostatistical and stochastic modelling can not be denied, conventional layer-cake modelling, within certain limitations, is a fairly reliable and cost-effective reservoir characterization tool. Introduction The two fields under study viz. Dikom-Madarkhat and Kathaloni-Bachmatia, discovered in 1990–91, are prolific producers and have their hydrocarbonaccumulations in thin, layered reservoirs inter-bedded with shale/mudstone of Eocene sandstones in Assam-Arakan geological province of North-East India. The production from these two fields presently account for around 25% of the total production (8400 tonnes/day) of the company. The basic objective of these studies was to build dynamic models to chalkout optimum field development plans and assess alternative exploitation schemes subject to their financial feasibility. Due to the heterogeneous nature of the reservoirs, sand-to-sand correlation was a major challenge and the problem was aggravated by sparseness of conventional cores, PVT and pressure transient survey results. However, the best possible correlation using 3D-seismic data, well logs, pressure-production and available well test analysis data was established for constructing the geological models which were eventually subjected to fluid flow simulation. This paper deals with these two case studies where conventional geological modelling for reservoir characterization was resorted to and satisfactory results were obtained thereof. Dikom-Madarkhat Field — Field Overview The Dikom-Madarkhat-Kuhiarbari oilfield established commercial oil production for the first time from the Lower Eocene formation in Upper Assam in April 1990 with the drilling of well N-438. Madarkhat, a satellite structure of the Dikom main structure and located in its south-west vicinity, was identifiedon the basis of close-grid 2D seismic data acquired during 1991–93 and commercial oil was discovered in well M-1 drilled in 1996. A review of close-grid 2D seismic data in Dikom-Madarkhat area revealed that Madarkhat structure is connected to the Dikom structure by a narrow north-south trendingridge. As seen from Figure 1, Kuhiarbari is another satellite structure developed to the east of Madarkhat structure with a minor intervening saddle.The first well Kuhiarbari 1 was drilled during October-November 2000 and produced clean oil on testing. A pressure transient survey carried out in December 2000 in this well indicated pressure level similar to that of D-15(360–365 ksc). The initial pressure in this well was of the order of the pressure of N-438 Block thus indicating communication with the main Dikom-Madarkhat structure. Till date, a total of 22 wells have been drilled in the Dikom-Madarkhat-Kuhiarbari field of which 18 wells have been completed as producers. Wells D-4 and 11 did not encounter any commercial hydrocarbon prospects whereas D-1and D-5 could not be completed as oil producers due to blowout and casing related downhole problems respectively. The remaining 16wells are currently producing at a total rate of around 1380 m3/daywith about 40% water cut.