Well Test Interpretation of Vertically Fractured Gas Wells

Abstract

In the analysis of field data it is important to bear in mind the proper relationship between the radial flow period and the linear proper relationship between the radial flow period and the linear flow period. The transition between these periods is quite long and can be misinterpreted as being the linear flow period. Another factor that can complicate analysis is turbulence. Introduction The trend in gas well testing has been to rely more on be early-time flow data of drawdown and buildup tests than on stabilized flow tests. The stabilized testing methods often are not adequate for complicated modem gas reservoirs because many of these reservoirs have extremely low permeabilities and the transient flow period of a well test can last for days or weeks. It has become common to augment the flow capacity of gas wells by hydraulically fracturing their producing formations. In deep reservoirs the induced producing formations. In deep reservoirs the induced fractures are generally vertical and tend to follow a single plane of weakness. The presence of a vertical fracture at the wellbore complicates the transient flow behavior of a low permeability gas well. The flow is further complicated when turbulence occurs near the wellbore. Russell and Truitt published transient drawdown solutions for vertically-fractured liquid wells. They developed methods of drawdown and buildup testing utilizing these solutions, which were based on numerical simulation. Clark applied the basic Russell-Truitt solutions to analyze fractured water injection wells by falloff tests. Field examples were given to substantiate the method of analysis. The Russell-Truitt solutions and well testing methods can be extended to gas well flow. Millheim and Cichowicz presented the drawdown equations for ideal gas flow, including the effects of formation damage and turbulence. Two actual field cases were presented, both of which exhibited extensive fractures presented, both of which exhibited extensive fractures and turbulent flow. For the pressure range of these well tests, the use of ideal gas equations was acceptable. These field cases were significant in being the first published data showing the occurrence of turbulence in fractured gas wells. We, also, observed such cases in practice. Our purpose here is to extend the theory of fractured gas well testing to the flow of real gases. To determine the effects of wellbore storage and turbulence on well test interpretation, we developed a finite-difference model to simulate well test conditions. Since gas wells usually are widely spaced and have high compressibility, the emphasis has been put on the early transient behavior before the effects of the outer boundary are noticeable at the wellbore. The Mathematical Model The geometry of our mathematical model is similar to that used by Prats. The well is centered in a circular uniform reservoir. A vertical fracture of infinite flow capacity penetrates the formation and passes through die wellbore. The wellbore itself is passes through die wellbore. The wellbore itself is not important. Fig. 1 shows a sketch of the problem. Fig. 2 shows the idealized model. JPT P. 625

A simple model of gas production from hydrofractured horizontal wells in shales

“…Most of the relevant papers have been devoted to short-and intermediate-time well testing: pressure build up, well storage effects, and flow tests. The classical papers by Prats (1961), Russell and Truitt (1964), Wattenbarger and Ramey (1969), Gringarten and Henry (1974) should be mentioned in this context. A 1-D (rectilinear) transient flow analysis was also applied to long-time production from hydrofractured wells in ultra-low-permeability formations.…”

Pressure-transient analysis of infinite-conductivity fractured gas wells producing at high-flow rates

“…Many authors (Wattenbarger and Ramey, 1969;Holditch and Morse, 1976;Guppy et al, 1982aGuppy et al, ,b, 1988Settari et al, 2000;Nunez et al, 2003;Hagoort, 2004) have stated that nonDarcy effects manifest itself as a positive skin factor, which is linearly proportional to the production rate; hence, it can be treated as ratedependent skin similar to non-fractured wells. Consequently, Eq.…”

“…One of first papers published on high-production rate effect on the behavior of fractured systems was by Wattenbarger and Ramey (1969). They concluded that non-Darcy effects caused by high-flow rate have more adverse impact on the gas flow behavior in the fracture than in the surrounding formation.…”

Analysis of Hydraulic Fracturing of High Permeability Gas Wells to Reduce Non-darcy Skin Effects

“…Holditch and Morse (1976) performed work involving non-Darcy flow in hydraulically fractured wells. Their work was a follow up on speculation by Wattenbarger and Ramey Jr. (1969) that turbulence within the fracture itself is sometimes more important then turbulence within the formation. Holditch and Morse's work was accomplished via numerical simulation and showed that for gas wells, nonDarcy flow within the fracture can have a significant effect on well productivity and pressure transient response.…”