Tight carbonate reservoirs, although less well understood and believed to require higher development costs and risks than conventional reservoirs, have become an important resource. Historically, tight reservoirs have been unpopular and unfavorable with geologists and reservoir engineers mainly due to difficulty associated with their development with no commercial productive value. Recently, with the increase in demand for oil and continuous development of new technologies, the time is right to carefully examine and develop such reservoirs. This paper discusses the development of one of the tightest carbonate reservoirs offshore Abu Dhabi, UAE. Initially, this reservoir was not planned to be developed based on the appraisal data collected. The average reservoir permeability is around 1 mD with a productivity index of 0.4 bod/psi. However, an intensive work was performed to evaluate the reservoir potential as being commercially attractive. Such process included gathering of additional data during the development of a major reservoir located below and the review of the core and test permeability data across the reservoir. Several model scenarios were created both on static and dynamic sides as well as an evaluation of the uncertainties and associated development risks. Effectiveness of the study took place when drilling horizontal wells across the most permeable intervals resulted in a production five times higher than expected. This resulted in a decision to embark on the field development and additional production data gathering for development optimization. Introduction The subject reservoir is a complex tight carbonate reservoir; it is an oil reservoir with a relatively large gas cap located on land, island, shallow and deep marine areas. It has been penetrated by several wells during the field development implementation of other reservoirs. Thus, it's well controlled in terms of structure, fluid contacts and porosity distribution also it has considerable proven GIP/OIP volumes. The reservoir is clearly dominated by lime-rich sediments with only a weakly effective micro-pore system. Locally, in the lower part of the Zone, framework pores within the Bacinella structures remain open. Small scale, hair line fractures associated with stylolites were observed in the upper part and may provide preferential flow. The cored wells resulted in a very low permeability values with an average of 1 mD. It was also production tested through DST and Rig-On-site (ROS) for a short duration in several wells. The test rate confirmed the tightness of the reservoir where in some wells; pressure test analyses were carried out and showed very low productivity index ˜ 0.4 bod/psi only within a maximum KH value of 44 mD-ft. However, a trend of permeability enhancement was observed towards the northeast of the field from cores and logs data. This trend was selected as the first to be considered for any future development plan. The main challenge in evaluating tight reservoir is how to identify the reservoir potentiality or deliverability to recover quantified reserve. Most of the tight reservoirs that have huge gas cap are developed for gas rather than oil rim, which is economically and operationally less attractive. Recently, with the increase of the oil demand and continuous development of new technologies, it was decided to develop the subject reservoir under depletion using a limited number of wells to contribute to a daily production rate of 500 STBOPD per well and also perform a feasibility study for full field development.
Development studies of a heterogeneous carbonate reservoir in Abu Dhabi-United Arab Emirates based on 10 years of data from eight oil producers and one pilot water injection well concluded that mobility enhancement of the injected phase would be a key factor of effective pressure support and recovery. Development studies of different options concluded that miscible hydrocarbon gas injection would give the highest recovery and was adopted for implementation. Variation of the reservoir fluid properties with presence of a gas cap at the crest was reported and the Minimum Miscibility Pressure (MMP) of the injected miscible hydrocarbon gas reported to be very close to the reservoir pressure. Simulation studies indicated that miscibility would not be sustained and gas percolation into the gas cap would always be there. The studies concluded that gas injection near the OWC would give better chance to benefit from gravity segregation of the percolated gas. Five spot inverted pattern at the crest in the area below the gas cap and staggered line drive WAG down flank in the highly under-saturated oil were selected as the optimum development option that was implemented in Phase I development area. All the development wells were completed using long radius open horizontal hole completion technique for both the producers and injectors. Hydrocarbon associated gas was processed and injected in its rich composition. Collected bottom hole samples after one year and half of injection indicated arrival of the miscible front into location of a newly drilled well offset to a gas injector suggesting successful miscibility. Project has been in progress achieving the target production plateau with some anomalies of high GOR. Surveillance plan was set to closely monitor front movement of both gas and water in key observation wells. This paper presents a case history of an early miscible hydrocarbon gas injection project in a newly developed heterogeneous carbonate reservoir and observations after four years of production and injection.
Full field development plan of a complex carbonate reservoir on land, islands, shallow and deep marine areas is a complicated issue at a high cost (building artificial islands, drilling from shore using highly deviated wells with long reach, etc.). This paper presents a multi-disciplinary approach for the full field development plan of the X field off coast Abu Dhabi. The reservoir under study was discovered in 1969 being an oil bearing reservoir with a thin gas cap. It has good porosity development (14–18 %) with low permeability (0.4 to 5 mD). Two strike-slip fault patterns (W-E and NW-SE) were mapped seismically. Image logs and dynamic data suggest that fractures are rare across the reservoir. The field exhibits a low relief structure (250 ft) with dips of less than 2 degrees down flank and a pool area closely to 500 sq. km. The reservoir shows lateral variation in fluid and petrochemical properties. A full field compositional model is being used to evaluate the field reservoir performance under various development scenarios. The field has been in Early Production Scheme (EPS) since early 1990 and has been on stream since December 2005. Due to reservoir tightness, coupled with the fact that the field is located on an environmentally sensitive area offshore Abu Dhabi, it is being developed with long horizontal wells drilled from clusters (land, island and artificial islands). Accordingly, most of the drilled wells are highly deviated and some of them are in the category of ERD (Extended Reach Drilling). For this reason, depth uncertainty associated with large departures and complex trajectories revealed to be an important component of the reservoir models uncertainty. Uncertainty analysis and risk mitigation were therefore tackled by the team as a priority for the field development. This paper addresses the uncertainty related to a low relief structure and the masking of shallow velocities, the depth uncertainty associated with high departure horizontal wells and the spatial petrochemical heterogeneity. Static and dynamic reservoir models have been built using information from wells and 3D seismic data. An integrated modeling approach involving Geology, Geophysics, Petrophysics and Geostatistics were applied to build the frameworks and property models. Both deterministic and stochastic techniques were used to populate the models with porosity, permeability and saturation. Multiple realizations were generated for the uncertainty analysis. Also discussed in this paper is the integrated work associated with the development plan optimization for this reservoir. A full field development plan was set using a 3-D compositional simulation model and calls for five spot gas injection patterns on the crest and staggered line drive on the flank using WAG injection. This required the drilling of 57 horizontal wells (23 injectors and 34 producers) in a shallow marine area. Subsequently, a detailed drilling strategy /development plan was set to develop the reservoir. The subject reservoir was put on production using a combination of Gas/Water/WAG injection schemes in late 2005 early 2006. Field History Location and Geological Aspects The field is located in the northeast part of Abu Dhabi, UAE. The field lies in a coastal marine area, which includes Land, Islands, Sabkha and Shallow-Marine environments (water depth varies from zero to eight meters) and Figure 1 displays the different depositional environments is the area. Being a very sensitive area concerning geological and wildlife issues, some concerns were raised at the time of the development in terms of environment and also some problems were faced while acquiring the 3D seismic during to a mix of shallow marine and land acquisition.
Field development of a heterogeneous carbonate reservoir in Abu Dhabi-United Arab Emirates focused mainly on the geosteering technique to penetrate the target level of the reservoir and drill and place long radius open horizontal hole for final completion as single producers or injectors. Tough drilling environment of natural islands, manmade islands, shallow and deep waters based on poor seismic data and depth uncertainties added to difficulties of predicting the setting depth of the 7" liner before geosteering operations of the open horizontal hole. Wells clustering to minimize drilling and surface facilities foot prints, top drive drilling using rotary steering system (RSS) techniques together with real time LWD helped to better control trajectory and placement of the horizontal holes as planned. Fluid identification in pilot holes as well as across the horizontal holes was successfully acquired to capture necessary reservoir fluid samples.Geosteering operations and the experienced difficulties of placing the horizontal holes as planned were affected by rock heterogeneity, rate of penetration, mud system, and real time monitoring and follow up. The implemetation of geosteering and real time data acquisition was supervised by the team abd resulted in timly decisions for proper action and achievement of the target development plan. However, depth uncertainties resulted in improper placement of three wells but these well were worked over to correct the location. In addition, coiled tubing accessibility problems have been experienced following production and injection commissioning. This problem was attributed to the undulating horizontal hole and the well completion technique. This paper presents a field case of drilling and geosteering of long radius open horizontal holes and wells completion. It points to possible reasons for experienced poor coiled tubing accessibility and the lessons learned for future development.
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