In a market continuing to experience commodity price uncertainty, operators looking to boost field economics turn to refracturing as a cost-effective alternative to drilling new wells. Hundreds of horizontal wells have been refractured in unconventional basins throughout North America to re-establish reservoir connectivity, target previously under- or unstimulated zones and restore production. Despite greater understanding of fracture geometry and innovations in downhole surveillance tools and diversion techniques, refracturing results have remained inconsistent.
Refracturing has long been used effectively in vertical wells. However, the geologic complexities of unconventional formations present unique challenges, starting with candidate selection. Refracture modeling helps manage economic risks by enabling engineers to better determine what is happening downhole and more reliably predict production results before capital is deployed. Modeling enables engineers to consider multiple refracturing scenarios and examine the impact on offset wells when a nearby well is refractured.
Refracture modeling remains problematic, primarily due to reservoir depletion, which results in decreased pore pressure and in situ stress, and lateral coverage challenges. Degradable chemical diverters, mechanical isolation methods and combinations of both have been used to improve coverage and connectivity. Because chemical diverters are bullheaded from the surface, it is difficult to know exactly where they go and how they affect ensuing fluid and proppant. Even with mechanical isolation, the fluid may travel down the annulus between the casing and formation to a previously depleted zone. A clear methodology for numerically simulating refractures would improve the efficacy of refracture modeling.
Numerical simulation methodology
To address depletion and lateral coverage issues, Schlumberger developed an integrated refracturing workflow that models refracture treatments in a numerical simulator to assess performance of previously refractured wells and better predict the performance of future refractured wells. As described in SPE 187236 “Proposed Refracturing Methodology in the Haynesville Shale,” the multidisciplinary workflow combines complex hydraulic fracture models, geomechanical models and multiwell production simulation. In its first implementation, it was used to optimize the refracturing strategy for a multiwell pad in the dry gas window of the Haynesville Shale.
At the heart of the workflow, the refracture numerical simulation methodology accounts for historical production depletion using calculated pressure and stress values along the lateral and in the reservoir. The altered stress fields resulting from reservoir depletion are then calculated through the workflow, which combines simulated 3-D reservoir pressure with a geomechanical finite-element model to quantify changes to the magnitude and azimuth of in situ stresses. The altered stress field provides input for modeling the new fracture system created by the refracturing treatment, which is validated by production history-matching data from a previously refractured well.
Divided into four main phases, the workflow is enabled by the Petrel E&P software platform to combine multiple disciplines and facilitate workflow standardization (Figure 1). The platform incorporates the Kinetix reservoir-stimulation- to-production software, integrating geophysics, geology, petrophysics, completion engineering, reservoir engineering and geomechanics to ensure data integrity. The software predicts the structure of the new fractures to enable a production forecast.