Effects of High Pressure-Dependent Leakoff (PDL) and High Process-Zone Stress (PZS) on Stimulation Treatments and Production
- Robert David Barree (Barree & Assocs. LLC) | H. Gordon Harris (University of Wyoming) | Brian Francis Towler (University of Wyoming) | Muthukumarappan Ramurthy (Halliburton)
- Document ID
- Society of Petroleum Engineers
- SPE Unconventional Resources Conference and Exhibition-Asia Pacific, 11-13 November, Brisbane, Australia
- Publication Date
- Document Type
- Conference Paper
- 2013, Society of Petroleum Engineers
- 2 Well Completion, 2.2.2 Perforating, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.8.1 Tight Gas, 4.1.2 Separation and Treating, 3 Production and Well Operations, 2.4.3 Sand/Solids Control, 2.5.2 Fracturing Materials (Fluids, Proppant)
- Economic cutoff for PZS, Shales, Process Zone Stress, Unconventional Reservoirs, Pressure Dependent Leakoff, Stimulation, CBM, Tight Gas
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Hydraulic fracturing continues to be the primary mechanism to produce hydrocarbons out of unconventional reservoirs like tight gas sands, tight coals and shale reservoirs. Over the last few decades it has been studied extensively. However, all the issues that arise during a stimulation treatment have not been understood correctly, yet, leading to costly trial and error approaches to fix them. Assuming that a majority of the perforations (or sleeves) are open and there are no issues with the stimulation fluids, screen-outs and/or pressure-outs during stimulation treatments in any type of reservoir can be attributed to either high pressure-dependent leakoff (PDL) or high process-zone stress (PZS). With high PDL the end result will be a screen-out if it is not addressed properly. However, with high PZS, it is the first indicator and in conjunction with fracture gradient and local stress environment one can understand the reasons for pressure-outs or screen-outs. With high PZS pressure-outs are more common than screen-outs. The objective of this work is to clearly explain and quantify these reservoir-related issues and once identified present solutions such that screen-outs and pressure-outs can be avoided in re-fracture and new well treatments. The effect of damage zone and the fluid lag or negative net stress zones and their contribution to the fracture tip effects will be presented. This work will also clearly show that zones that exhibit high PZS (greater than 0.20 psi/ft), irrespective of the formation type, are economically poor producers.
The tools for identifying these reservoir-related parameters include a diagnostic fracture-injection test (DFIT) and a grid-oriented fully functional 3D fracture simulator with shear decoupling. The relationship between high PZS and the local stress environments and their contribution to issues during a stimulation treatment are presented based on the analysis of 3000 plus DFIT's from the Rockies. Coal, tight gas sand and shale formations are part of the 3000 plus DFIT dataset presented in this work. Examples from coal and shale formations presented earlier by the author are referred in this work. Finally, guidelines (Ramurthy 2012) are presented such that stimulation treatments in high PZS zones that contribute to poor production can be avoided and high PDL zones that lead to good production can be optimized, thereby saving completion costs.
Hydraulic fracturing has been an integral part of the oil and gas industry for decades. The recent surge in the unconventional reservoir completions would not have been possible without hydraulic fracturing. The horizontal well development in very low permeability shale plays like the Niobrara, Eagle Ford and Mancos are dependent upon hydraulic fracturing to be economical. However, optimization of these hydraulic fracturing treatments to avoid screen-outs and achieve maximum proppant pack conductivity to tap these hydrocarbons is an ongoing process. One of the common problems hindering this optimization process is the lack of understanding about pressure dependent leakoff and its effects and the focus on the fracturing fluids to find a solution for screen-outs. Additionally, consistently (and incorrectly) blaming near wellbore issues for high treating pressures before diagnosing the issues also impedes the optimization of stimulation treatments to achieve maximum recovery. The basic concepts behind hydraulic fracturing and understanding its deficiencies can help explain fracture tip effects (i.e. process zone stress) better as well as explain the reasons behind the fracture models under predicting the net pressures.
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