Abstract
The objective of this paper is to investigate how to acquire fracture closure pressure data accurately and cost-effectively in shale development activities. If the injection volume is small during a hydraulic fracturing job, it is a common practice to consider the ISIP (Instantaneous Shut-In Pressure) as a proxy of fracture closure pressure, which generally requires a long falloff period to evaluate. However, ISIP should consist of variable parameters such as the original closure pressure, net pressure, stress shadow effect, and mid-field fracture complexity.
In this paper, we introduce the consecutive DFITs (Diagnostic Fracture Injection Test) approach. Closure pressure is evaluated by falloff pressure analysis after a micro fracturing injection operation, and pore pressure could also be evaluated if the falloff period is long enough. This micro fracturing operation is generally called DFIT. In order to evaluate how the ISIP, closure pressure, and pore pressure change from stage to stage, we performed DFITs consecutively at the sequential hydraulic fracturing stages in a horizontal well drilled in the Eagle Ford shale.
The consecutive DFITs revealed that the ISIP gradually increased up to certain level from fracturing stage to stage as expected. However, the observed closure pressure was almost constant in the sequential stages, which was against our expectations. In addition, the evaluated pore pressure was also almost constant. Initially we expected that closure pressure would increase because of the uplift due to the stress shadow effect. Since the consecutive DFITs showed the same closure pressure in each stage, we concluded that stress uplift could disappear before the fracture closure in next stage or that the stress shadow had little impact on the closure pressure and the pore pressure in next stage under the current fracture design. On the other hand, the ISIP could be affected by the stress shadow in the short term or by the mid-field fracture complexity becoming higher than the previous stage. The correlation between the ISIP and the closure pressure was established with these consecutive DFITs results. Although the gap between the ISIP and closure pressure varies from stage to stage, it was confirmed that the correlation, with some uncertainties, could be used to estimate the closure pressure within an acceptable range.
This paper presents the Eagle Ford case study, which confirmed how accurately ISIP can determine closure pressure considering multiple factors. There are hydraulic fracturing operations in huge number of horizontal wells in the shale development. Therefore, the correlation built by consecutive DFITs is useful because that correlation can provide operators with the confidence to optimize the completion design based on the ISIP which can be obtained at a low cost.
