Modeling and Analysis of Hydraulic Fracture Skin and Its Control on Shale Gas Production From Horizontal Wells
Abstract
It is widely recognized that hydraulic fracturing creates a region of altered permeability near the fracture/matrix interface. Although we have a limited understanding, it is believed this region plays an important role on the production rates. The objective of this study is to model the region as a fracture skin explicitly including local physical and chemical phenomena and their associated effects, namely: (1) permeability improvement due to unpropped fracture development during the fracturing, (2) stress increase due to slick water invasion and clay-swelling effect developing after the fracturing, and (3) water-saturation buildup due to capillary end effect (CEE) developing during the flowback and production. A sensitivity analysis on the derived skin indicates key parameters controlling production and help in minimizing the damage.
In order to develop the hydraulic fracture skin model, two cases are considered: In Case 1, gas flow occurs without any alterations in the formation (no skin), whereas Case 2 considers gas flow occurs in a region near the hydraulic fracture/matrix interface in the presence of the alterations. The difference in the derived pressure drop between the two cases (i.e., Case 2 minus Case 1) is the total pressure drop due to the three combined effects. Improvement in permeability is introduced in Case 2 using a stress-sensitive matrix permeability model. This model considers the presence of unpropped fractures and cracks in the altered zone. Formation damage due to CEE is introduced by defining a new associated gas relative permeability, while damage due to clay swelling is introduced by adding an osmotic pressure term into the permeability model.
The proposed fracture skin increases at a decreasing rate with distance away from the fracture/matrix interface, which mirrors the invaded fracturing fluid (slick-water) near the interface. The sensitivity analysis performed shows that clay swelling is the predominant damage mechanism and is controlled by the geo-mechanical parameters of the permeability model: (i) the normalized effective stress at the fracture matrix/interface when clay swelling develops in the altered zone, and (ii) strength of the unpropped fractures and cracks inside the damage zone to stay open. On the other hand, the contribution of CEE to the skin is mainly controlled by the average water saturation in the damaged zone. CEE can thus be severe in formations with high water saturation. To minimize the damage, the salinity of the injected water needs to be increased, which in turn will reduce the chemical imbalance between the clay-bound water and slick-water, preventing osmosis. In addition, using proppants with a wider particle size distribution could reduce the damage by allowing smaller size proppants to invade into these secondary fractures and cracks, hence, keeping them propped open during the production.
The hydraulic fracture skin model can be used with the commercial simulators. In this thesis, using Eclipse, I show that the model helps to gain a significantly improved understanding of the alterations induced by the fracturing fluid invasion. This improved understanding will help in minimizing the damage and will ensure optimum benefits of improved productivity.
The literature addresses the issues related to wellbore skin in-depth but the wellbore skin is negligible in unconventional wells and it is the hydraulic fracture skin that counts during the production of oil and gas using horizontal wells with multi-stage fractures. In the latter case, the total fracture surface area of the well could be in the order of hundred acres. An incorrect description of the skin enveloping this large area could be critical in analyzing production trends and optimizing the well performance. The skin is incorrectly identified by the industry as the “stimulated zone”, not considering the impact of water invasion on the transport. This article is the first documented effort looking into the hydraulic fracture skin in a systematic fashion considering its multi-physics nature.
My work focusses on shale gas wells. It is important that the analysis and discussion is extended into shale oil/condensate wells. Intuitively, one would expect that these effects persist during oil and gas multi-phase flow because the problem is still drainage of water during the production leading to CEE, whereas the clay swelling is a phenomenon independent of the hydrocarbons. However, it should be recognized that clay swelling is less pronounced in gas reservoir because of their high thermal maturity and compaction, which reduces clay-bound water.
Subject
Hydraulic Fracture SkinFracking
Clay Swelling
Capillary End Effect
Shale Gas
Unconventional Resources
Citation
Atama, Grace Pepe (2022). Modeling and Analysis of Hydraulic Fracture Skin and Its Control on Shale Gas Production From Horizontal Wells. Master's thesis, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /197271.