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SPE Members Abstract The low invasion coring method was used to provide highly reliable connate water saturation in the Oligocene age, Lingshui III sandstone, Yacheng 13–1 reservoir, development Wells A1 and A2. Development well coring was implemented to resolve an uncertainty in the total gas-in-place. A synthetic oilbase mud was designed to simultaneously operate at the 350 F wellbore temperature, provide reliable core connate water saturation, insure high quality well logs and minimize formation damage. The polyalphaolefin (PAO) synthetic base oil is biodegradable and is less damaging to the environment than diesel or mineral oil. Procedures for obtaining a low invasion core in a remote location are described. Careful planning and execution are necessary for successfully obtaining low invasion core. This is the first use of low invasion oilbase mud coring in China. Water saturations from cores are essential to accurately calculate the gas reserves. Possible gas-water contact variations cause uncertainty in connate water saturation from capillary pressure. Salinity variations in the reservoir complicate the calculation of connate water saturation from electrical logs. Accurate water saturation and salinity data from cores are needed to develop log models that can be applied in non-cored wells. Introduction The Yacheng 13–1 Natural Gas Field lies in 300 feet of water 60 miles south of Hainan Island in the Ying Ge Hai West contract area, South China Sea, People's Republic of China. Figure 1 shows the field location. The field was discovered by ARCO China, Inc. in July 1983. The productive interval is Late Oligocene/Early Miocene age sandstone which onlaps a basement high (the Yacheng Spur) to the west and is truncated by a regional unconformity as shown in Figure 2. The offshore extension of the Red River Fault system parallels the west side of the Yacheng Spur separating the Yacheng Field to the east from the down-thrown Miocene age Ying Ge Hal basin to the west. The reservoir is in the Lingshui III Formation (13–1 sandstone) which lies along the eastern flank of the Spur. The Lingshui III Formation is made up of a series of stacked fluvial-dominated, tidally influenced, estaurine sand-rich deposits >85% net to gross) capped by a thick, very coarse-grained sandstone interpreted to have been deposited in a progradational shoreface/mouth bar environment. Reservoir thickness ranges from zero feet at the unconformity updip to over 700 feet downdip Total gas column is on the order of 1600 feet with a gas/water contact at -12993 feet subsea observed in Well 13–1A. Six delineation wells were drilled from 1983 to 1990. With the signing of gas contracts for 330 mm scf/d, development drilling began in 1994. Figure 3 is a structure map showing the locations of the six initial development wells. Wells 13–1-A1 and A2 are the first two development wells drilled prior to field start-up. These wells were cored with synthetic oil-base mud using the low invasion method to provide connate water saturation for the calculation of gas reserves and depletion planning. Well logs from earlier drilled delineation wells indicated average water saturation was 36.1 % pv using the Waxman-Smits interpretation method, whereas capillary pressure measurements made on delineation well core samples indicated average water saturation was 29.1 % pv. This corresponds to a difference of 10% in total gas-in-place. Coring was implemented to resolve this discrepancy. YACHENG CORING PLANS Successful low invasion coring requires the following equipment and procedures: P. 335 |
