Factors Affecting Production Behavior in Tight Gas Reservoirs

Abstract

Abstract Tight Gas Reservoirs (TGR) are one of the primary types of unconventional reservoirs to be exploited in the search for longlasting resources.TGR's are difficult to produce for a number of reasons. Due to their low productivity a thorough understanding is needed regarding the factors that affect gas production rate over the life of these reservoirs. This paper is focused on analyzing the effects of drainage area, gas rate, fracture conductivity, porosity, and reservoir permeability on production performance. In particular, the impact of permeability, from ultra tight (0.0004 md) to tight (0.1 md) reservoirs, on drainage area and reserves is analyzed in detail. A semi-analytical simulator is used in this study. A conceptual case study was performed comparing a hydraulically fractured vertical well with a multiple-fracture horizontal well in same reservoir. Fracture conductivity is estimated by using StimLAB proppant consortium correlations for different flow rates, which takes into account non-Darcy pressure drops and other factors. The results of this work conclude that in ultra tight reservoirs, the drainage area is significantly reduced. Only the near fracture rock is drained, and a high density of wells and fractures is needed. This behavior changes with increase in reservoir permeability. In ultra-tight reservoirs, horizontal wells with multiple fracs may be the only viable option for commercial production. A general workflow is also described as to how forecasting in such reservoirs can be made more accurate. Introduction With the increasing demand for oil and gas, and declining production from existing fields, conventional hydrocarbons cannot meet the current and future energy demands of the world. The oil and gas industry is investing in the development of methods and technologies needed to exploit unconventional resources. Fig-1 shows the resource triangle (Holditch 2006; Hein 2009) and describes how unconventional reservoirs are different from conventional reservoirs in regards to technology required for commercial exploitation and reservoir volumes. Compared to conventional reservoirs, unconventional reservoirs are larger in volume but difficult to exploit. In the resource triangle as we move towards unconventional resources, better technology in drilling and development are key factors to success. Unconventional resources include tight gas reservoirs, gas hydrates, Coal bed methane, and oil shales. This paper is focused on tight gas reservoirs (TGR). The term Tight Gas is commonly used for referring to low permeability reservoirs producing dry natural gas (Holditch 2006). Though the definition of Tight Gas Reservoirs is different in different parts of the world, there is general agreement that permeability below 0.1 mD characterizes tight gas reservoir. In some countries, for purposes of gas sale pricing, TGR can have permeability upto 1 and even 2 md. In this paper we look at permeabilities below 0.1 mD.