Integrated Development Plan for a Naturally-Fractured Gas-Bearing Granite Reservoir

Hazlett, W.G. (Gemini Solutions, Inc.) | Snow, P.W. (Mosbacher Energy Co.) | Von Gonten, W.D. (W.D. Von Gonten & Co.) | Glasgow, W.M. (Mosbacher Energy Co.) | Banks, R.J. (Mosbacher Energy Co.) | Duncan, C.S. (Worley International, Inc.)

OnePetro 

Abstract This paper describes the characteristics of and development plans for the PY-1 concession, a gas reservoir contained in a natu rally-fractured granite formation. Over a several-year period, exploration and delineation wells were drilled offshore India. The paper covers all aspects of the integrated geoscience/engineering development study, including seismic, core and fracture analyses, well logging, well testing, and reservoir simulation. In addition, we include the offshore facilities, pipeline and onshore compression plans. The develop ment plan was prepared by a team of engineers and geoscientists made up of both operating company personnel and contract and consulting personnel. The result of the study is a development plan for a granite reservoir. We conclude that highly-fractured granite formations should not be overlooked as potential reservoirs. This granite reservoir should provide a steady gas supply to any one of several commercial manufacturing facilities planned for the area. Introduction and Exploration History The PY-1 Field is located in the northern offshore part of the Cauvery Basin, Southeast India. The field is located offshore at a location 18 kilometers due east of the town of Porto Novo and 30 kilometers southeast of the town of Cuddalore. Water depth varies, but most of the field is in less than 75 meters of water. The PY-1 field was discovered by the India's Oil and Natural Gas Corporation Ltd. (ONGC) in 1980. subsequent to the discovery, ONGC drilled a number of wells on the structure and identified it as a fractured granite basement gas reservoir. In 1995, a 25 year Production Sharing Contract was signed between the Government of India and the PY-1 Consortium currently made up of the following partners: Mosbacher India, L. L. C., Operator, Energy Equity Corporation and Hindustan Oil Exploration Company. In 1997, the Consortium drilled an appraisal well, PY-1–12, in order to perform a long term produc tion test to establish reservoir performance and areal extent, obtain conventional core of basement reservoir for reservoir characterization, obtain quality logs for reservoir characteriza tion, and establish low known gas depth for the basement reservoir. In the remainder of this paper, we detail the results of a comprehensive study to determine the potential of this field and the facilities necessary to profitably produce the reserves. Geophysics Four 2D seismic surveys have been acquired since 1976. Some of these have been reprocessed. Vintage data quality varies between poor and good. The Top of Basement reflector occurs between 1.4 and 2.4 seconds two-way travel time in the majority of the PSC area, however, reflectors up to 4.0 seconds occur on the downthrown sides of the major bounding faults. A number of wells in the area have velocity data that is available. Average velocity to Basement is approximately 2050 mps. Geology The structure is sealed by Cretaceous to Eocene aged shales. The productivity of the reservoir comes from intense natural fracturation, brecciation and some matrix porosity influenced by massive regional intersecting faults bordering the PY-1 Conces sion Area. The PY-1 Field is a gas accumulation within a Precambrian granitic reservoir on the crest of basement ridge known as the Porto Novo Horst. This NE-SW oriented ridge is an offshore extension of Kumbakonam-Madanam-Shiyali Ridge. Intense fracturing and brecciation of the PY-1 Field basement reservoir rock is attributed to wrench tectonics during Cretaceous and Tertiary periods. The productive interval is contained in the top of a granitic basement high which has been deeply weathered, highly fractured and brecciated to form the reservoir. The high amount of fracturing has created sufficient porosity and perme ability for excellent gas reservoir quality.

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