After-Closure Analysis To Identify Naturally Fractured Reservoirs

SPE Annual Technical Conference and Exhibition

Sharma

2017

Over the past two decades, Diagnostic Fracture Injection Tests (DFIT), which have also been referred to as Injection-Falloff Tests, Fracture Calibration Tests, Mini-Frac Tests in the literature, have evolved into a commonly used and reliable technique to evaluate reservoir properties, fracturing parameters and obtain in-situ stresses. Since the introduction of DFIT analysis based on G-function and its derivative, this method has become standard practice for quantifying minimum in-situ stress and leak-off coefficient. However, the pressure decline model that underlies the G-function plot makes two distinct and important assumptions: (1) leak-off is not pressure-dependent and, (2) fracture stiffness (or compliance) is assumed to be constant during fracture closure. In this study, we first review Nolte's original G-function model and examine the assumptions inherent in the model. We then present a new global pressure transient model for pressure decline after shut-in which not only preserves the physics of unsteady-state reservoir flow behavior, elastic fracture mechanics and material balance, but also incorporates the gradual changes of fracture stiffness (or compliance) due to the contact of rough fracture walls during closure. Analysis of synthetic cases, along with field data are presented to demonstrate how the coupled effects of fracture geometry, fracture surface asperities, formation properties, pore pressure and wellbore storage can impact fracturing pressure decline and the estimation of minimum in-situ stress. It is shown that using Carter's leak-off is an oversimplification that leads to significant errors in the interpretation of DFIT data. Most importantly, this article reveals that previous methods of estimating minimum in-situ stress often lead to significant over or underestimates. Based on our modeling and simulation results, we propose a much more accurate and reliable method to estimate the minimum in-situ stress and fracture pressure dependent leak-off rate.

Section: Fig 1 Diagram Showing Sequence Of Events Observed In a Dfitmentioning

confidence: 99%

New Variable Compliance Method for Estimating In-Situ Stress and Leak-Off from DFIT Data

SPE Annual Technical Conference and Exhibition

Sharma

2017

“…The methodology of analysis used for various flow regimes is discussed in detail by Soliman et al (2005). Chipperfield (2006) and Uribe et al (2007) published extensions of the after-closure-analysis methodology. Chipperfield expanded on work published by Houze et al (1988).…”

Section: Brief Review Of After-closure Analysismentioning

confidence: 99%

“…2 and 3. The equations used in the calculations were presented by Bourdet and Gringarten (1980) and Bourdet et al (1983) and were summarized very well by Chipperfield (2006).…”

Section: Fets In a Dual-porosity Systemmentioning

confidence: 99%

Application of After-Closure Analysis to a Dual-Porosity Formation, to CBM, and to a Fractured Horizontal Well

Soliman

Miranda

SPE Production &Amp; Operations

2010

Summary Published techniques for after-closure analysis of fracturing data usually assume the presence of a vertical fracture intersecting a vertical well. In addition, these published techniques usually assume that the formation is homogeneous. When the formation is naturally fractured or the well is horizontally intersecting a transverse vertical fracture, those assumptions are obviously violated and the published analysis techniques might not be applicable. Through the use of analytical and numerical solutions and application to actual field data, this paper investigates the analysis of after-closure data for heterogeneous formations, a naturally fractured formation, a CBM, and a fractured horizontal well. This paper briefly reviews the various available techniques for after-closure analysis, pointing out the strengths and weaknesses of each. An analytical solution for an injection-falloff test for a naturally fractured formation has been developed and is presented. This solution might be used to analyze data for a minifrac test in a naturally fractured formation where the fracture has healed. Numerical simulation validated the developed solution for a fracture that has healed. The same numerical simulator was used to expand the solution to a situation where the fracture maintains residual conductivity. The solution for a minifrac test in the case of a transverse fracture is also presented and discussed. Using a numerical simulator, minifrac tests are simulated and analyzed for both heterogeneous formations and fractured horizontal wells. Guidelines for the analysis of such data have been developed and presented. Field data are also presented. One case presents a minifrac test for a heterogeneous formation. A case for a transverse fracture intersecting a horizontal well is also presented and analyzed. A third case for CBM is discussed.

“…G-function derivative analysis requires a plot that includes the bottom-hole pressure, the pressure derivative (dP/dG), and the superimposed pressure derivative (GdP/dG). The characteristic curved shape of the pressure derivative and the superposition pressure derivative is used to identify the leak-off behaviour, of which there are four common types, i.e , normal leak-off behaviour, pressure-dependent leak-off from opened natural fractures, fracture-height recession during shut-in, and fracture-tip extension (McLellan and Janz, 1991; Barree and Mukherjee, 1996; Craig et al, 2000; Chipperfield, 2006; Cai and Dahi Taleghani, 2021). Plahn et al (1997) proposed a numerical model of the fracture pump-in and flow-back test, which gives a better estimate of fracture closure pressure than the aforementioned conventional methods.…”

Section: Introductionmentioning

confidence: 99%

Complex Fracture Closure Pressure Analysis During Shut-in: A Numerical Study

Energy Exploration & Exploitation

et al. 2022

Conventional fracture closure models typically assume homogeneous formation. This assumption renders complex fracture closure pressure analysis during shut-in insufficient at present. In this paper, we use a cohesive zone method (CZM)-based finite element model to obtain the fracture closure pressure and minimum horizontal principal stress of the major fracture and the branch fracture based on pressure fall-off analysis. Key factors including leak-off rate, injection time, and stress anisotropy are discussed in detail. A quantitative relationship between fracture closure pressure and these factors is plotted to reduce or eliminate the errors caused by conventional fracture injection diagnostic models. The results show that leak-off rate, injection time, and stress anisotropy have a significant effect on the fracture closure process. Fracture closure in naturally fractured formations is a slow process, and the early closure pressure represents the closure of the branch fracture, which is much higher than the minimum horizontal stress. This investigation provides new insight into how to estimate the in-situ stress in naturally fractured reservoirs.