Industry Article: Scientific Approach Applied to Multi-Well Pad Development in Eagle Ford Shale

As the inventory of single-well pads in North American shale plays continues to build, the industry needs to determine: 1) What is the optimum spacing for an in-fill well; 2) Where should new multiple in-fill wells be drilled? An engineered approach enhances play economics and ensures maximized recovery with the least capital expenditure.

Publication: World Oil
Publication Date: 07/01/2017

by Kush Gakhar, Dan Shan, Yuri Rodionov, Raj Malpani, E. A. Ejofodomi, Jian Xu, Karsten Fischer, and Timothy Pope, Schlumberger

Optimizing well spacing, placement configuration and stimulation design are key issues that need to be solved, as the industry enters the next phase of unconventional reservoir development. The design and evaluation strategy for these second-generation wells, which will be drilled next to depleted sections of the reservoir, requires a deeper understanding to minimize unproductive overlap and maximize recovery.

Eagle Ford Lithology

The Eagle Ford has good reservoir-quality rock, but the challenges associated with completing in-fill wells persist. The Eagle Ford has five distinct lithostratigraphic sections (A–E). The B-unit is subdivided into two separate categories. The B1-B2 unit (Donovan 2010) has high total organic carbon (TOC) content, whereas the B3-B5 zones have a higher content of smectite and kaolinite ash beds. To develop a fully optimized multi-well pad strategy, it is critical to account for these ash beds, because they impede vertical fracture conductivity, by creating pinch points when hydraulic fractures propagate through weak formation bedding planes in sections with a higher frequency of ash bed occurrence.

Unconventional fracture model

The Eagle Ford is naturally fractured, and within these planes of weakness, shear failure occurs during hydraulic fracturing. An unconventional fracture model (UFM) was developed to simulate this failure mechanism, to understand the complex interaction between hydraulic fractures and natural fractures.

The UFM simulation showed crossing occurs, if the compressive stress acting perpendicular to the frictional interface is sufficient to prevent slip at the moment when the tip of the hydraulic fracture contacts the interface, and the induced stress on the opposite side is sufficient to initiate a tensile fracture.

The model solves an array of equations governing fracture deformation, height growth, fluid flow and proppant transport, in a complex fracture network with multiple propagating fracture tips. In comparison, the traditional hydraulic fracture evaluation technique through planar fracture modeling overestimates fracture length and ignores the impact of natural fractures.

UFM can be integrated into a reservoir-centric software platform that combines hydraulic fracture modeling with dynamic reservoir simulation and geomechanical finite element modeling, that form an essential part of the workflow study.

Single-well pads

In the Eagle Ford, there is a high inventory of single-well pads that has led to significant pressure depletion near the parent well. This pressure drop alters the in-situ stress field. Reduction in the magnitude of the principal horizontal stresses is accompanied by re-orientation of the stress field to maintain geomechanical equilibrium. These in-situ stress changes have considerable impact on the hydraulic fracture geometry generated at offset, second-generation “child wells”.

Depleted pressure sinks created from parent well production lead to asymmetric hydraulic fracture growth from child wells. This asymmetric fracture behavior is detrimental to both parent and child wells. Parent wells can suffer from "frac hits," which often result in production loss and the need for wellbore cleanout.

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