Chad Development Project consists of three fields (Kome, Miandoum, and Bolobo) in the Doba basin. Completions in the Upper Cretaceous reservoirs must address the following characteristics:Unconsolidated permeable sands (0.5 < k > 20 Darcies).Large average particle sizes (100 to 1,000 microns).Broad particle size distributions (uniformity coefficient-d10/d50-ranging from 3 to 8+).Sand layers separated by highly reactive shale.Low-gravity, high-viscosity crude oil (30 cp to 400+ cp).Separate oil/water contacts for different sand intervals.Completion interval lengths spanning a few tens of feet for a single sand to several hundred feet commingling several sands. It is a challenge to design an efficient and reliable sand-control completion for these characteristics. Individual sand-control completion designs were evaluated by a multidisciplinary completion team (geology, reservoir, completion, and production engineers). To date, five types of sand-control completions have been used in the Chad project: high-rate water packs (HRWPs), frac packs (FPs), frac packs using shunt tubes (FPw/Shunts), FPw/Shunts and an isolation Packer (FPw/shunts&IP), and openhole stand-alone screens (OH-SAS). Only cased hole completions are addressed in this paper. When this paper was prepared, 51 sand-control completions had been installed in Miandoum field. Initial postcompletion well performance was measured using short duration (24- to 48-hr) cleanup and test periods. A significant learning curve for completion quality and operational efficiencies was overcome by both operations and engineering teams working together closely to improve techniques. This paper will describe the technical and operational hurdles as well as the results achieved with various completion methods. Introduction The Chad Development, which consists of three fields, is located in southern Chad approximately 430 km south of N'Djamena (Fig. 1). This paper will focus on the Miandoum completions, where the majority of work has been completed. The first oil production from Miandoum to fill surface facilities and the pipeline commenced on July 24, 2003 as this paper was being prepared.
Formation damage due to fines migration after the onset of water production presents a major technical challenge for many sandstone reservoirs around the globe. Oftentimes, significant productivity impairment is observed shortly after water breakthrough. This is particularly true for the Chad Doba basin lower "M" and "A" Sand reservoirs where studies have shown that a major contributor to this damage mechanism is the fluid velocity near the wellbore. As a result, exponential decline in productivity index is typically observed over very short periods.To arrest the productivity impairment, various completion techniques were evaluated for ways to reduce the velocity of the produced fluids near the borehole. Typical completion designs employed to date have been cased hole gravel packs (including frac-packs) which enhances the velocity profile of the well as produced fluids converge to the perforations. Maximizing the reservoir to the wellbore interface reduces the velocity profile and conceptually prolongs the onset of formation damage caused by fines migration. Based on that conceptual model, openhole completion techniques were evaluated for feasibility. Upon analyzing the geology of the selected candidate, it became apparent that fracturing the formation was possible as it was a relatively thin amalgamated sand package. As a result, the openhole frac-pack concept became a practical option as it results in the lowest velocity completion possible by maximizing the reservoir surface flow area.Openhole frac-pack completion activities were executed in mid-2009 and well productivity has been sustained even after water breakthrough. This paper reviews initial openhole frac-pack design concepts, execution lessons learned, and well productivity performance. Doba
Interstitial-water studies on samples collected during DSDP Leg 50 indicate that saline brines were advected through aquifers into the deeper sedimentary strata. Maxima in the magnesium concentration at about 500 meters at Site 415 and 600 meters at Site 416 support the hypothesis that high-magnesium silicates (palygorskite and sepiolite) are formed in situ in an environment conducive to silicate formation, i.e., in strata where silica recrystallization occurs.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractA Gulf of Mexico deepwater subsea development project required seawater injection for pressure maintenance of the producing reservoir. Typical water injector performance in the Gulf of Mexico has been characterized by rapidly declining injectivities that could be partially restored by frequent acid treatments. This type of performance was unacceptable for this field development plan, which required one subsea water injector capable of injecting up to 25,000 BWPD continuously for up to five years.Two changes to typical Gulf of Mexico water injector completion design were incorporated: 1) an openhole frac pack completion, and 2) a unique downhole filter system. These changes resulted in improved water injector performance compared with that typically experienced in the Gulf of Mexico. Initial injectivity was excellent and remained so over the entire injection period. No acid stimulations were required to either initiate or maintain injection. This paper presents the design and installation aspects as well as the actual performance results for this water injector.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractSeveral sand control completions in the initial development of a Gulf of Mexico deepwater turbidite field failed prematurely when placed on production. After an extensive multi-discipline review of completion designs and procedures, the cause of these completion failures was determined to be an inverted sand-shale stress contrast, that is, shales were at lower minimum horizontal stress than adjoining sand layers. Direct evidence of this sand/shale stress reversal was obtained from mechanical strength property data derived from dipole sonic logs in two wells in this field.Frac pack completion design was modified to account for this sand/shale stress reversal by incorporating perforation standoff from adjacent shales and by using smaller pad volumes. The modified design was successful in achieving excellent frac packs in seven subsequent completions in this field. Long-term performance of these modified frac pack completions has been excellent.
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