Rate Dependence of Transient Linear Flow in Tight Gas Wells

Abstract

Many tight gas wells (permeability less than 0.1 mD) exhibit transient linear flow; sometimes for several years. This behaviour differs from radial flow in many ways. This paper reports another import difference between linear flow and radial flow? rate sensitivity. It has been shown and accepted for years that real gas pseudopressure can be used to apply analytical solutions to transient radial flow. However, it has been noticed that analytical solutions can be in serious error when applied to transient linear flow. Specifically, the slope of the √t plot departs from the analytical value as the flow rates or degree of drawdown becomes higher. This paper demonstrates the rate/drawdown sensitivity of transient linear flow. Then a correction factor is presented which corrects the slope of the plot and improves the accuracy of √k Ac and OGIP, as calculated from production/pressure performance. Introduction Many wells in tight gas reservoirs have long-term performance which exhibit only linear flow, not radial flow, during the transient period. Wells have been observed which stay in the transient linear flow regime for several years. Some of these wells have hydraulic fractures and some do not. It is usually not practical to analyze tight gas wells with build-up tests, but long-term production and pressure data can be used for analysis. Previous papers have presented methods of analysis(1–5). The analysis of these wells comes from plotting [m(pi) - m(pwf)]/ Qg vs. √t and observing the slope, mCPL, and the end of the straight line, tesr (end of the transient linear flow period). From these values, √k Ac and OGIP can be calculated. It has long been accepted that radial flow transient solutions can be approximated by analytical solutions, in terms of m(p), regardless of flow rate. Constant rate solutions have been emphasized, but it can also be shown that constant pwf flow can also be approximated by analytical solutions, regardless of the level of drawdown(5). The drawdown/rate dependency linear flow is different than analysis of radial flow. This difference was demonstrated with reservoir simulation in this work. A correction method was developed to improve the accuracy of analysis of transient linear flow for analyzing tight gas wells. Effect of Drawdown on Transient Linear Flow (Constant pwf) A number of transient linear flow cases were run with constant Pwf. It was found that these cases did not have the same slopes as the analytical solution, but varied according to the level of drawdown. In order to demonstrate this effect, a dimensionless drawdown parameter is defined as follows: Equation (Available In Full Paper) Some results are plotted in Figure 1. It can be seen that the slope of these plots is greatly affected by the level of drawdown, DD. As the drawdown value increases, the slope (mCPL) value decreases. Since the analysis equations(5) are based on analytical solutions, the calculations may be wrong when actual data is analyzed.

Production data analysis of tight gas condensate reservoirs

“…(5) can be simplified to Eq. (6), which is exactly the same equation derived for dry gas (Ibrahim and Wattenbarger, 2006). One should note that the appearance of a constant "2" in the numerator of Eq.…”

“…Several studies (Ibrahim and Wattenbarger, 2006, Poe, 2002, Miller et al, 2010and Nobakht and Clarkson, 2012 focused mainly on the flow of single phase gas to obtain the linear flow parameter, x f √k. Ibrahim and Wattenbarger noted that the transient linear flow solution, derived for liquids, required the use of pseudopressure and empirical drawdown factor to account for changing gas properties with pressure.…”

On the Liquid-Flow Analog To Evaluate Gas Wells Producing in Shales

“…Modifications and extensions to the analog have been suggested over time, as in Agarwal (1979) who introduced the concept of pseudotime to consider applications to buildup tests in which the gradient in the reservoir at the start of the test is large and in Finjord (1989) who provided a framework for this concept. The long-standing interest in applying concepts proposed in AlHussainy et al (1966) to fractured wells producing at a constant pressure, in which gradients may indeed be large, has led to additional constructs and includes, among others, empirical corrections as in Ibrahim and Wattenbarger (2006); use of weighted average values of the viscosity, l, and compressibility, c g , rather than such values at the initial pressure as in Poe (2002) and Miller et al (2010); and extensions of the pseudotime concept to flowing conditions in linear reservoirs (see Nobakht and Clarkson 2012 and references cited therein).…”

Shale Gas-in-Place Calculations Part II — Multi-component Gas Adsorption Effects

“…It is the foundation for the determination of reserves specifically in low permeability reservoirs where flow is in transient regime for very long periods of time, sometimes months or even years (Ibrahim andWattenbarger, 2006, Ambrose et al, 2011). Since production involves transient flow for such long periods, it is critical to have a best estimate of the resource ─both liquid and gaseous phase hydrocarbons in-place─ so that the uncertainties can be alleviated during the reserve estimation.…”