Tight Gas Surveillance and Characterization: Impact of Production Logging

Abstract

In this paper, we discuss the importance of production logging in tight gas reservoirs due to the large number of commingled entries that are typically associated with tight gas wells. We discuss the challenges associated with obtaining high quality production log data in tight gas reservoirs and the subsequent production log interpretation. Along with addressing these challenges, some initial production logging results are presented that highlight the importance of geology. These results include:Most of a well's production comes from a few zones,Some observed production rates are substantially higher than anticipated based on matrix properties, andIn a few cases, sands that can be locally correlated between wells may be associated with anomalously high production rates. Introduction Interest in unconventional resources is increasing in order to meet the world's growing demand for energy. Unconventional gas resources include tight gas, shale gas, and coal bed methane. The National Petroleum Council (Raymond et al. 2007) reported that there are an estimated 4,024 Tcf of global natural gas resource in tight gas reservoirs and 8,225 Tcf in coalbed methane reservoirs. This paper will focus on tight gas, which is often defined as gas in low permeability rock that must be stimulated in order to obtain commercial flow rates. Tight gas reservoirs pose many challenges to both reservoir engineers and geoscientists. Many of these challenges result from the poor reservoir connectivity due to the low permeabilities and the limited size of some of the sand deposits. Some interesting questions for optimal tight gas development include:How is the reservoir depleted? andWhat geologic features impact water and gas productivity? Since many tight gas developments involve stimulating multiple zones and commingling the production in a common wellbore, understanding the wellbore inflow profile is required to answer these questions. (In this paper, zones are defined as perforated and stimulated sandy intervals identified from their gamma ray signature. Zones may consist of one or more closely spaced sands.) Inflow profiles are traditionally obtained via production logs. However, obtaining high quality production log data in many tight gas wells is difficult due to the large number of closely spaced zones, relatively low flow rates, and water-gas slugging. In the next section, we will provide a field overview. After the overview, we will discuss the challenges with production logging in tight gas wells. The following section will provide some initial results that highlight the importance of geology in the Piceance Creek Unit. The last section will summarize the results. Background on the Piceance Field and Development The Piceance Creek Unit (PCU) was formed in 1940 and is located in northwestern Colorado approximately 15 miles northwest of Rifle. Figure 1 shows the location of the PCU (outlined in black). Recent completions have targeted the Mesaverde formation (see Fig. 2). The development of the Mesaverde in the Piceance basin poses many challenges due to its geologic complexity, low permeability, and thick gross interval.