Fracture Surface Area Estimation from Hydraulic-Fracture Treatment Pressure Falloff Data

Abstract

Summary It has often been reported that the peak production of a well drilled in tight formations is highly dependent on the fracture-contact area. However, at present, there is no efficient approach to estimate the fracture surface area for each fracture stage. In this paper, we propose a method to calculate the fracture surface area on the basis of the falloff data after each stage of the main hydraulic-fracture treatment. The created hydraulic fracture closes freely before its surfaces hit the proppant pack, and this process can be recognized in the pressure falloff data and its diagnostic plots. The pressure-decline rate during fracture closure is mainly caused by the fluid leakoff from the fracture system into the formation matrix. For a horizontal well drilled in the same formation, with the known leakoff coefficient(s) and fracture-closure stress(es), the total-fracture surface area can be calculated for all stages to meet the requirement of the fluid-leakoff rate. The wellbore-storage effect, friction dissipation, and tip extension dominate the early pressure falloff data. Whereas the transient pressure dominated by friction losses typically lasts approximately 1 minute, the tip extension might end after approximately 15 minutes. Therefore, falloff data should be acquired for at least 30 minutes to observe a fracture-closure trend. The fracture-closure behavior can be identified on the G-function plot as an extrapolated straight line or on the Bourdet derivative in log-log plot as a late-time unit slope. The behavior of the late unit slope depends on the pressure-decline rate, or correspondingly, to the fluid-leakoff rate. Therefore, the total-fracture surface area can be estimated using hydraulic-fracture design input values for the formation-leakoff coefficient and fracture-closure stress. The calculated fracture surface area represents the combined area of primary and secondary fractures—effectively all fracture surfaces contributing to the fluid leakoff. We applied the approach to all stages in a horizontal well that exhibit the fracture-closure behavior. The approach shows some promise as a potential way to estimate fracture surface areas that could allow an early estimate of the expected well performance.

Unconventional Well Test Analysis for Assessing Individual Fracture Stages through Post-Treatment Pressure Falloffs: Case Study

“…There were also no precise measurement technologies for recording production rate and pressure during each stage to evaluate each cluster or stage's contribution. The pressure falloff data tool has garnered significant interest in the oil and gas industry since it is a powerful method for defining prime fracture parameters to gain insight into the effectiveness of the treatment jobs' fracture [9].…”

“…The same concept was used to analyze the pressure falloff data by Liu et al [9], who determined the total fracture surface area for both natural fractures and hydraulic fractures on a stage-by-stage basis. In this paper, we modified the workflow, which enables us to calculate fracture half-length and identify the main fracture flow regimes after the treatment of the Meramec Formation, STACK Play of the Anadarko Basin, Oklahoma.…”

“…High-pres drawdown may also cause formation rock compression, which leads to a reductio matrix permeability with changes in reservoir pressure or stress [3,4,8]. The product of unconventional wells heavily relies on the effective fracture contact area or the prop fracture area per cluster, which is critical in evaluating hydraulic fracture treatment formance [9]. Figure 1.…”

“…Most of the previous post-treatment analysis studies of unconventional reservoirs do not provide quantitative discussions with detailed support case studies, and very few field studies have discussed the application of pressure falloff data [9]. We present guidelines to better understand unconventional well test analysis through critical literature reviews and case studies of fracturing treatment analysis to address the lack of quantitative evaluation.…”

Advancement of Hydraulic Fracture Diagnostics in Unconventional Formations

“…Lately, PTA was conducted in readily available fall-off data after each stage completion to estimate the created SRV during the fracturing process. This method provides a free and realtime investigation method to estimate the fracture geometry and a measure of completion efficiency [140,141]. However, the results from these techniques are still under debate and open more room for research.…”