Evolution of a Hybrid Fracture/Gravel-Pack Completion: Monopod Platform, Trading Bay Field, Cook Inlet, Alaska

Abstract

Summary Placing a small fracture job before an inner-casing gravel pack has evolved as the preferred completion for some reservoirs in Cook Inlet, AK. The hybridcompletion give the theoretical benefit of higher initial rates provided by thefracture and continuity of production provided by sand control. Introduction Simple perforated completions of the 1970's yielded poor long-term resultsat the Trading Bay field, Cook Inlet, AK. Permeability problems were blamed;therefore, limited-zone fracture work was problems were blamed; therefore, limited-zone fracture work was performed on several wells. Whether the fracture work contributed performed on several wells. Whether the fracture work contributed more to sand control than to linear flow benefits an issue in thelate 1990's. To examine this concept, intervals were fracture stimulated without gravel packing with full-zone, high-slurry-concentration treatments with much shorter fracture lengths than in earlier work. These jobs have yielded superior results, although sand entry may occur later in the life ofthe wells. Next, conventional gravel packing with and without subsequentfracture work, formerly considered too difficult operationally, was tired, withgood results. Thus evolved a hybrid process that involved use of a small fracture to prepack perforations before gravel packing. The first two wells that were fractured before being gravel packed have been successful. Cleanliness concepts from gravel packing are incorporated in the fracture procedure. A one-trip fracture/gravel-pack procedure now being pursued has tremendous potential for synergy in procedure now being pursued has tremendous potential for synergy in terms of high initial rates combined with consistentproductivity and significant reduction in the cost of applying both techniques. Many years of sporadic development provided the data leading to implementationof the new completion technique. Several wells have been completed with variouscombinations of hydraulic fracturing (conventional and high slurry density), gravel packing, and simple perforating. It is believed that a one-stepfracture/ gravel-pack will be the norm for gravel packing in the future in manyfields, particularly those with short pay zones (less than 100 ft). This paperout-lines the development of the completion and discusses each of its components. Recent job details and early evaluation of results are also presented. Recent success of the work has allowed planning for the complete rehabilitation of the Monopod, a single-logged, 32-slot platform set in 1966. This work can be adapted to other Cook platform set in 1966. This work can be adapted to other Cook Inlet platforms, and the hybrid completion is believed tohave application worldwide in augmenting productivity from wells requiring sand control in medium to tight formations. Reservoir Character The subject intervals have been producing oil from the Cook Inlet's Trading Bay field since 1966 when the Monopod was set. The field is a complexly faulted structure located in the Upper Cook Inlet basin of Alaska. Until recently, development emphasized the Miocene Lower Tyonek sands and the locally prolific Miocene Hemlock conglomerate. Shallower sands, designated as the Tyonek A and Bintervals, have always been known to be productive. These sands have been completed periodically since 1968 in 13 wells, but completions were short-livedwill stable production rates were low (about 50 BOPD). The Tyonek A and Bintervals of 20 sands ranging in thickness from 5 to 100 ft, separated bycoals, and producing oil of 18 to 21 degrees API with a paraffin content producing oil of 18 to 21 degrees API with a paraffin content of 3 to 5%. History of Completions In total, 13 wells were completed in the Tyonek A and B zones in severaldifferent fault blocks. Initial rates of 200 to 1,350 BOPD were encouraging, but very rapid decline rates were experienced, unlike the other Tyonek sands. Before 1990, only 3 of 12 wells were productive for more than 2 years. Allother wells were tested at noncommercial rates or declined to uneconomic rateswithin 12 months. Paraffin content was originally assumed to be the cause oflow productivity and extensive research was conducted to solve wax deposition problems. Seven organic-solvent treatments performed to alleviate suspected waxproblems met with limited performed to alleviate suspected wax problems metwith limited success. Wells were also reperforated and subjected to diesel back flushes to mitigate near-wellbore damage, but these treatments also metwith limited success. Gradually, fill problems also were recognized, but they were not sufficiently severe to suggest an immediate need for sand control. After 10 years of fighting wax problems, emphasis shifted to the apparentproblem of fines migration. Several Tyonek A and B wells were recompleted with conventional hydraulic fracture work. At the time use of gravel packing wasrejected because the technique's effectiveness over extended intervals was doubted. It was felt that fracture work would minimize formation fluid velocities more effectively. Conventional Fracture Work The fracture work pursued was rather innovative in that the subject formation is poorly consolidated and permeabilities are believed to range from10 to 100 md. The original fracture work involved fairly high sand loadings (upto 10 to 12 lbm/gal of 20/40-mesh U.S. sand) after conventionally ramping upfrom 0.5 lbm/gal. The net effect was to place long, dun fracture wings (on thetheoretical order of 300 ft long and one or two sand grains wide) with high slurry concentrations near the perforations. The middle one-third of the payinterval was routinely perforations. The middle one-third of the pay interval was routinely perforated with the idea that the anticipated upward growth ofthe perforated with the idea that the anticipated upward growth of the fracturewings would facilitate vertical coverage. The completions were initiallysuccessful and were given the diminutive name of "frac-pack" because conventional fracture work generally involves much larger sand volumes.(Further examples of frac-pack work range from these fracture jobs without gravel packing to prepacking perforations before pumping gravel packs to worksimilar prepacking perforations before pumping gravel packs to work similar tothe hybrid fracture/gravel-pack discussed in this paper. Thus, the term"frac-pack" is not used here because it has been used too loosely within industry.) The fracture work was successful in yielding initial monthlystabilized rates of 200 to 300 BOPD, typically a three-fold increase over unstimulated production. In most cases, however, these fracture jobs failed within several months (Fig. 1). Fracture work was discontinued after several failures. In subsequent years, fill was cleansed out of the subject wellboreswith coiled tubing. Production rates could be re-established almost to early post fracture rates through these cleanouts. Its became clear that, althoughfracture work could increase production rates substantially, fill influx was present and a means of sand control was to allow long-lived completions. SPEPE P. 395