Comprehensive Evaluation of Fractured Gas Wells Utilizing Production Data

Abstract

The ability to determine the effective half-length and conductivity of hydraulic fractures is important for estimating a well's long-term production performance. The determination of these properties, along with the formation effective permeability, can result in more accurate predictions of the ultimate hydrocarbon recovery. In very low permeability reservoirs, fracture half-length is the key to optimum reservoir development. Being able to quantify these properties also allows for improved understanding of the effects of treatment design changes. Post-treatment pressure buildup testing has been the most common method for determining the effective length of hydraulic fractures. One of the major drawbacks of the pressure transient test in very low permeability reservoirs is the extremely long shut-in time required to observe a sufficient amount of the fractured well transient behavior to properly characterize the formation and fracture properties. This long shut-in time is undesirable due to the fact that the well is not able to produce and generate revenue during this time. This paper reports on the most recent results of an ongoing study of the production performance of hydraulically fractured wells. The focus of the study is a comparison of the performance of conventionally fractured wells and those that have been completed with the treated water and low proppant concentration ("waterfrac") technique. A new evaluation technique for comparing the effectiveness of the treatments utilizing production data is introduced. The advantages and limitations of the production data analysis technique are discussed, as well as an improved understanding of the results of waterfrac treatments in low permeability gas reservoirs. The use of a comprehensive suite of analysis techniques for the production performance of fractured wells to obtain estimates of fracture half-length, fracture conductivity and formation effective permeability is detailed. Specialized diagnostics, performance history matching with analytic solutions and specialized type curve analyses have been used for several areas to estimate the fracture and formation properties from the bilinear, formation linear and pseudo-radial flow regimes. Introduction The practice of pumping waterfracs has spread to a wide range geographically. The results have been mixed in that the technique works better in some areas than in others. Waterfracs must be evaluated both on technical (scientific) and economical merit. For example, the best economic solution may not always be the best from a technical standpoint. The explanation of how the waterfrac treatments work has been limited to the theory of fracture conductivity created by surface asperities (mismatches in the geometry of the fracture faces upon closure). The small concentrations of proppant pumped could also concentrate at points of reduced fracture width creating, in effect, a wedge that is able to support and keep the fracture open to a certain degree depending on the magnitude of the in-situ stress and the properties of the proppant. Original publications related to the usage and results of waterfrac treatments pumped in the Cotton Valley sandstone formation in East Texas were published beginning in 19971–3. Since then methods to evaluate the success of the waterfrac treatments compared to conventional designs have been limited mainly to the direct comparison of well productivity. In many cases no adjustment was made for the varying conditions under which the wells were produced. Compensation for difference in flowing tubing pressure, initial reservoir pressure, differences in reservoir quality, etc. were generally not taken into account and left to the discretion of each individual looking at the comparison. The lack of accounting for these parameters is often due to the fact that the data is not readily available.

Evaluating Stimulation Effectiveness in Unconventional Gas Reservoirs

“…Information on type curve analyses can be found in the early work of Fetkovick 1980 and Cinco-Ley 1981 and more recent work of Blasingame 1991 andAgarwal 1999. Additional information on the development and application of type curve analyses is provided by England 2000, Pratikno 2003, Mattar 2003, Rushing 2005, Ehlig-Economides 2006, Anderson 2006, and Crafton 2006. The production profiles for the various reservoir simulations were evaluated to identify production signatures that could be used to characterize the "relative" conductivity of the primary fracture and of the network.…”

Proving It - A Review of 80 Published Field Studies Demonstrating the Importance of Increased Fracture Conductivity

“…70 Previously published analyses did not consider that compression was added to the field which benefited the newer waterfracs. Before correcting for this error, waterfracs appeared to show 16% lower production than conventional fractures in 12-month cumulative production.…”

An Integrated Single-Well Approach to Evaluating Completion Effectiveness and Reservoir Properties in the Wind Dancer Field

“…Multi-Layer-fracturing models (pseudo-3D models) and Planar-3D models (full 3D) are utilized for modeling fracture propagation 6,7 . Stages with single sand fracture stimulation were the initial focus to ensure that evaluation is simple i.e.…”

“…When production logs are available, production allocation techniques using pressure traverse calculation that honor material balance are used to create a unique pressure and rate history for the individual zones. Rate-transient analysis techniques 6,7 can then be used to obtain quantitative estimates of the reservoir effective permeability, effective fracture half-length, and average fracture conductivity (provided certain flow regimes or limits of the flow regimes are exhibited in the production data). This together with production history matching techniques is used to estimate reservoir and fracture properties using analytical and numerical simulators (Figure 4).…”