A New Refracture-Candidate Diagnostic Test Determines Reservoir Properties and Identifies Existing Conductive or Damaged Fractures

Abstract

Abstract A new refracture-candidate diagnostic test is presented that requires a brief injection at a pressure exceeding the fracture initiation and propagation pressure followed by an extended shut-in period with the pressure falloff recorded. Provided the time of injection is short relative to the reservoir response, the pressure falloff can be analyzed as a slug test by transforming and plotting the falloff data on a variable-storage, constant-rate drawdown type curve for a well producing from single or multiple finite- or infinite-conductivity vertical fractures in an infinite-acting reservoir. Characteristic variable storage behavior is used to diagnose a pre-existing fracture retaining residual width and to determine if a pre-existing fracture is damaged. Using the proposed model and analysis methodology, a quantitative type-curve/model match can be used to estimate reservoir properties. In addidtion to the new fracture diagnostic solution, a new single-phase fracture-injection/falloff dimensionless pressure solution is also provided along with new pressure-transient solutions for a well producing from multiple arbitrarily-oriented finite- or infinite-conductivity fractures. Introduction Oil and gas wells often contain potentially productive layers bypassed either intentionally or inadvertently during an original completion.Subsequent refracturing programs designed to identify underperforming wells and recomplete bypassed layers have sometimes been unsuccessful in part because the programs tend to focus on commingled well performance and well restimulation potential without thoroughly investigating individual layer properties and the refracturing potential of individual layers. Perhaps the most significant impediment to investigating layer properties is a lack of representative and cost-effective diagnostics that can be used to determine layer permeability, reservoir pressure, and to quantify the effectiveness of previous stimulation treatments. Post-frac production logs,[1–2] near-wellbore hydraulic fracture imaging with radioactive tracers,[3–4] and far-field microseismic fracture imaging[5] all suggest 10% to 40% of the layers targeted for completion during primary fracturing operations using limited-entry fracture treatment designs are bypassed or ineffectively stimulated. Quantifying bypassed layers has proved difficult because imaged wells represent a very small percentage of all wells completed. Consequently, bypassed or ineffectively stimulated layers may not be easily identified, and must be inferred from analysis of the commingled well stream, production logs, or conventional pressure-transient tests of individual layers. A refracture-candidate diagnostic used prior to a refracture treatment should complete the following objectives.[6]To determine if:A pre-existing fracture exists.A pre-existing fracture is damaged.To estimate:Effective fracture half-length of a pre-existing fracture.Fracture conductivity of a pre-existing fracture.Reservoir transmissibility.Average reservoir pressure. When the diagnostic test objectives are achieved, the benefits of refracturing can be easily evaluated, and the incremental production from a refracture treatment can be predicted. Quantitative conventional pressure-transient testing, which includes drawdown, drawdown/buildup, or injection/falloff tests at a pressure less than the fracture propagation pressure, can be used to achieve the objectives of a refracture-candidate diagnostic test. However, conventional pressure-transient tests are best suited for evaluating a single layer. For wells producing from multiple layers, multilayer pressure-transient tests have been published,[7] but in practice, determining layer flow rates for test interpretation from multiple layers is problematic—especially with upwards of 20 layers producing.[8] In general, a cost-effective quantitative diagnostic test does not exist for wells producing from multiple layers. Diagnostic testing in low permeability multilayer wells has been attempted, and Hopkins et al.[9] describe several diagnostic techniques used in a Devonian shale well to diagnose the existence of a pre-existing fracture(s) in multiple targeted layers over a 727 ft interval. The diagnostic tests included isolation flow tests, wellbore communication tests, nitrogen injection/falloff tests, and conventional drawdown/buildup tests. The post-frac diagnostic program described by Hopkins et al.[9] is thorough and addresses the objectives of a refracture-candidate diagnostic. However, the diagnostic program is also expensive and time consuming - even for a relatively simple four layer case. Many refracture candidates in low permeability gas wells contain stacked lenticular sands with between 20 to 40 layers which need to be evaluated in a timely and cost effective manner.