Field Application of Water-Coning Theory to Todhunters Lake Gas Field

Abstract

Todhunters Lakefield has multiple gas reservoirs with a moderate-to-strong water drive that requires careful production practices to maintain water-free producing rates and maximum recovery of reserves. This paper describes the completion methods and the application of the Muskat-Wyckoff water-coning theory to help maintain maximum efficient producing rates. Introduction Significant increases in water production in a gas well producing from a water drive reservoir may result from producing from a water drive reservoir may result from one of two causes if no mechanical problems exist. Either the well is watering out because of its structural location, or the attempted gas withdrawal rates are too high and water is being coned into the well. Both cases have been experienced in Todhunters Lake field. For the case of watering out because of structural position, volumetric calculations of gas remaining in position, volumetric calculations of gas remaining in each of the fault-block reservoirs can be used in conjunction with geologic maps to monitor the rise of the gaswater contact. The problem of water coning is more difficult and was not recognized in the early field history because of the complex geology and lack of reservoir data. With increased development it became apparent that water problems were occurring even in wells with good structural position. An early study concluded that the gas wells could be coned prematurely if not monitored carefully. The study also concluded that water-free production rates calculated by the Muskat-Wyckoff theory production rates calculated by the Muskat-Wyckoff theory in conjunction with the graphs presented by Arthur compared favorably with rates calculated using a company-developed numerical coning model. This was initially a surprise in view of the many Muskat-Wyckoff simplifying assumptions. Subsequent field application of the theory and evaluation with a commercially available numerical coning model have confirmed the results of the earlier numerical coning model study. The practical implications of this comparison are significant. This means a tool is readily available for quickly establishing water-free production rates and calculating reserves in this particular field, which has an active water drive and a particular field, which has an active water drive and a rising gas-water contact. To calculate reserves in a multiwell reservoir with 15 to 20 years of production is difficult and expensive using any currently available numerical coning model. Field History Todhunters Lake gas field is located in Yolo County, Calif., about 7 miles west of Sacramento. Fig. 1 shows the field outline as determined by the California Div. of Oil and Gas. To date, 35 dry holes and 50 production wells have been drilled within the field. The discovery well was drilled by Tidewater Oil Co. (now Getty Oil Co.) when Reavis and Baker No. 1 (Section 33) was completed for 5,360 Mcf/D in May 1967. The area had been explored quite heavily before Tidewater's discovery, but no significant gas shows had been encountered other than in Atlantic-Getty IOC No. 1 (Section 33). The complicated geology has resulted in a field of producers sandwiched between dry holes. By Jan. 1, 1975, some 29,695 MMcf of gas had been produced from the field. Of this total, the main producing produced from the field. Of this total, the main producing sands have been the Starkey One (S-1) with 10,419 MMcf cumulative production and the Starkey Four (S-4) with 7,133 MMcf cumulative production. Fig. 2 shows the production curve for the field. A breakdown of the cumulative production by sands is shown in Table 1. The field ranked fourth in annual production of all the northern California gas fields in 1974. JPT P. 552