This paper continues the investigation of interwell fracturing interference for an infill drilling scenario synthetic case based on Eagle Ford available public data and explores pressure and stress-sink mitigation strategies applied to the simulation cases developed in the previous publication (SPE 174902). Emphasis is given to refracturing scenarios, given the intrinsic restimulation value for depleted parent wells and the strategic importance due to the current low oil prices.
The stress and pressure depletion methodology is expanded in this paper, adding a refracturing scenario before the infill child well is stimulated. Changes in stress magnitudes and azimuths caused by new and reactivated fractures are calculated using a finite element model (FEM). After refracturing the parent well, modeling shows that stress deflection and repressurization of the originally depleted production zone will reduce subsequent fracture hits from infill wells.
The first mechanism to reduce fracture hits is the stress realignment, which promotes transverse fracture propagation from the infill well away from the parent well. The second fracture-hit-reduction mechanism is repressurization of depleted zones to hinder fracture propagation in lower-pressure zones. Prevention of fracture hits by active deflection results in an increased stimulated reservoir volume (SRV) for both the parent and child wells. Overall pad level and individual wellbore cumulative production experience a significant increase due to increased SRV. With proper reservoir and geomechanical data, this approach can be applied in a predictive manner to decrease fracture-hit risk and improve overall recovery.
This workflow represents the first comprehensive multidisciplinary approach to coupling geomechanical, complex hydraulic fracture models, and multiwell production simulation models aimed towards understanding fracture-hit reduction using refracturing. The workflow presented can be applied to study and design an optimum refracturing job to prevent potentially catastrophic fracture hits during refracturing operations.
This paper describes the successful combination of different stimulation techniques during the development phase of an offshore field in Congo. A total of four wells were drilled and completed during 2008-10. The target reservoir, the Sendji carbonates, has a total thickness of 150-170 vertical meters with an interlayer of ten meters. It is composed of silty-shaly dolomitic and quarzitic sandstone with interbeds of shale and sand grading to sandstone and silt with dolomitic cement characterized by very poor petrophysical properties. Laboratory testing with cuttings showed that formation is more than eighty percent soluble with fifteen percent hydrochloric acid. The typical well has an average azimuth and inclination of 60 and 350 degrees respectively, in the target reservoir. A good quality cementation job of the casing was required in order to ensure isolation from the aquifer. The well completions are both open and cased hole multistage fracturing completion systems. Selected zones of varying lengths were hydraulically fractured by using proppant / seawater-based borate crosslinker fluid or by using twenty percent delayed, viscoelastic and straight acid. Two wells were stimulated by means of proppant fracturing while the other two by acid fracturing. Due to upper / lower water zones with no bounding beds, it was important to define formation mechanical properties for fracturing designs in order to avoid fracturing into water bearing layers. A Mechanical Earth Model (MEM) was created by using sonic anisotropy measurements, modular dynamic test results, and other common electrical logging data together with drilling parameters. The detailed study of design methodology, different stimulation fluids and operation sequences are described. Particular attention will be given to the comparison of well responses to multi-stage proppant and acid fracturing techniques.
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