Orienting Live Well Perforating Technique Provides Innovative Sand-Control Method in the North Sea

Abstract

Summary The Varg field is a development in the North Sea operated first by Saga Petroleum and currently by Norsk Hydro. It consists of a not-normally-manned wellhead platform (WHP) with dry wellheads and a floating production storage offloading (FPSO) vessel for processing the crude. The reservoir is heterogeneous sandstone of Upper Jurassic age, with estimated reserves of approximately 35 million STB. During development planning of the field, it was felt that sand production was likely to occur, and this phenomenon was actually experienced during well testing. Because this development was considered to be a marginal field, an extensive program was initiated to review and select a technical completion solution that would minimize the risk of sand production so that production potentials could be maximized. This paper discusses how a unique perforating solution was developed to provide the controls needed to manage the potential for sand production. The first part of this paper summarizes (1) the drainage strategy, and the implications of that strategy on selecting the perforation intervals, and (2) the results of a rock mechanics study that focused on the conditions critical for the onset of sand production, as well as recommendations for minimizing the risk. The primary recommendations were to avoid perforation of the weakest intervals and to shoot perforations that would be oriented parallel to the maximum horizontal stress. The second part of the paper describes how successful oriented perforating was achieved in these wells. The primary completion challenges included the following:Development of a slickline system that could perforate the entire zone simultaneously in a given direction in a vertical well.A means to remove the perforating guns from the well without having to kill it after completion of the perforation, as there was insufficient rathole to leave the guns in the hole. An oriented modular perforating system was developed so that these conditions could be achieved. The requirements were met by installing a gun anchor in the well before the completion was installed. Then, the orientation of an orienting lug was measured with a gyro, and, based on the data from the survey, the guns were aligned in the correct orientation. Subsequently, gun sections of length maximized to available lubricator length were run on slickline. The gun section aligned itself automatically to the measured orientation of the orienting lug because of the equipment design. Currently, three wells have been perforated using this system. All of them are sand-free since startup 1 1/2 years ago. Introduction The Varg field is located in the Norwegian sector of the North Sea, close to the Maureen field in the U.K. sector (Fig. 1). The reservoirs consist of an Upper Jurassic sandstone in a faulted anticline with an underlying salt ridge. The fields cover an area of 7 km2, and the depth to the top of the reservoir is at 2,720 m below sea level. The field is complex. The mapping of faults and the extent of the different sand units is uncertain; the different exploration wells show different oil/water contacts (OWC's) and reservoir pressures. Field development was approved in May 1996, and the selected development solution consisted of a WHP with dry wellheads. Production is manifolded on the WHP and routed to the FPSO through two subsea flowlines. Sand-monitoring devices are installed on the flowlines of each production well, and on test and first-stage separator inlets on the FPSO. The development solution is illustrated in Fig. 2. A jackup rig is used for drilling and completion work; when the drilling program has been completed, the rig will be removed from the platform. Fig. 2 shows the jackup rig in place. A total of four exploration wells have been drilled, and two of these have been re-entered and completed as oil producers. In addition to these two wells, a total of five developmental wells have been drilled. Drilling activities are currently ongoing, and the position of drilled wells is illustrated in the top reservoir map shown in Fig. 3. Production Strategy The reservoirs contain undersaturated oil with underlying water. The reservoir pressure varies somewhat between segments and is approximately 345 bar, with a temperature of 128°C. A typical log section for the reservoir sand in Well A is given in Fig. 4. The reservoir is separated into an upper sand unit and a lower sand unit by a 40-m-thick silt layer. The selected depletion strategy employs pressure maintenance by injection of produced gas plus supplemental injection of treated seawater in a water-alternating-gas drive. Pressure maintenance is required to (1) maintain miscibility between the reservoir oil and injected gas, (2) allow high production rates at increasing water production, and (3) help minimize the potential for sand production. All wells penetrate a layered system with predicted low vertical communication. The high-permeability sand layers may act as thief zones for water and gas if perforated and, thus, will offer a high risk for sand production. The approach described below to define perforation intervals for Well A has been adopted for all production wells. Two scenarios of perforating the upper, oil-bearing sand of Well A are illustrated in Fig. 5. The first is given as a "perforate all" case (identical to the exploration well-test interval), and the second is a "restricted perf" case, which shows the well-perforation intervals for optimized production. For the optimization of perforation intervals, it was necessary to find a good compromise between maximizing the well productivity and minimizing the risk of sand production and early water and gas breakthrough. These needs were to be addressed by avoiding the perforating of the high-permeability layers. Fig. 5 illustrates this tradeoff, where two production scenarios are compared for Well A:Case 1-Perforate all sands/maximize oil rate.Case 2-Restrict perforation intervals/minimize sand-production risk, and delay water production. Although there is some difference in productivity in the simulated models, the case with restricted perforations shows considerably lower rates of water and gas production.