Abstract

Well stimulation techniques like hydraulic fracturing, which can involve large financial investments, call for a basin- or reservoir-specific approach to maximize production. Integrated solutions use a performance-based process that integrates petrophysical and reservoir characterization expertise with production and completion knowledge by developing and refining more complete interpretation and completion models based on comprehensive data. This process includes the current service standard of design, execution, and evaluation, but goes far beyond basic well stimulation, which has historically used limited data. Another important feature of the integrated solutions is a proper risk assessment based on available data. Often, especially in old fields, information is out of date, limited, or unavailable. Combining together available pieces of information through the structured process helps put together the "big picture," which subsequently provides the support for engineering decisions.

Case histories discussed in this paper illustrate how distributed teams of experts use improved models to develop solutions for old wells and future development that focuses on ultimate productivity throughout well or reservoir productive life, regardless of boundaries between various technical disciplines. Cases of application for fracturing in Nurzhanov field, Kazakhstan, are discussed.

Introduction

A total of seven wells were selected for hydraulic fracturing in the Nurzhanov field, all of which have been treated. Triassic formations in Kazakhstan were not considered as a target for hydraulic fracturing before. Part of it is contributed to low success of hydraulic fracturing on other formations, including Jurassic horizons in the same Nurzhanov field, and also to general considerations of poorly understood formation deliverability. Fracturing treatments discussed in this paper were urged to prove or disprove the formation potential.

Thirty percent of the wells in this formation were contributing 70% of the total production; increased flow from the rest of the wells would greatly improve the field performance.

Field Description

The Nurzhanov oil-gas-condensate field is located in the Embensky region of Atyrau province, 170 km south-southeast of Atyrau city. The structure was evaluated for deep drilling by seismic exploration in 1958. Exploratory drilling started in 1963. The field was discovered in 1964. Exploration work was carried out in 1964 to 1978. Tectonically it is hosted in a salt dome structure. Nine horizons have been delineated: VIII1 (gas-condensate), VIII2 and VIII3 (gas-oil), and VIII4 in the Callovian; IX1 (gas) and IX2 (gas-condensate) in the Middle Jurassic; II-T, III-T, and VI-T (gas-oil) in the Triassic. Jurassic productive horizons are found at depths of 2179-2320 m and Triassic horizons at 3109-3308 m.

All stimulation treatments were performed in Triassic formation. The height of the oil part of this reservoir totals 36–75 m. The height of the gas part equals 56 m. The water/oil contact has been established at a level of 3168-3404 m. The reservoir is fully or partially stratified and tectonically screened. The productive horizons are composed of terrigenous rocks, the host rocks being porous. Open porosity in the host rocks accounts for 14-19%, and permeability equals 15-60 mD. The oil-saturated thickness totals 7-11 m, and the gas saturated thickness amounts to 19 m. The oil saturation factor totals 0.51-0.67 while the gas saturation amounts to 0.44-0.64. The initial formation pressure totals a maximum 34.9 MPa, and static temperature 92-96oC. The density of the oil is 823 to 899 kg/m3. The oil contains 0.33 to 1.46% sulfur, 2.05-4.22% paraffin, 15.66% resin, and 1.12% asphaltenes.[1]

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