Gurley, Jack, Northern Natural Gas Producing Co., Wichita, Kans.
In a given gas-producing area, production histories of numerous gas wells are analyzed and the productive capacity at any given time related, on a percentage basis, to the well's best month on the line. In the case of gas wells that are produced at full capacity against line pressure throughout their producing life, the best month usually occurs in the first month of production. Each month's production is then related to the best month as a "per cent of best month".This "per cent of best month" capacity is average and plotted against time for various groups of wells, comparisons are made between the groups and they are then summarized into one master productivity projection on this same basis. This productivity projection is converted into an income projection for wells with various initial producing capacities. The resulting graphical plot is a family of curves with cumulative gross income plotted against months on the pipeline with the initial 30-day average producing capacity as the parameter. Though the study covers only one particular area, the possible application of the method to other areas is discussed, as is the possibility of further refinement of the method through a relationship based on short-term flow tests.
To formulate long-term economic predictions related to gas reserve exploitation, it is necessary to predict the long-term withdrawal rates physically possible for each well. While gas-well deliverability and performance predictions have been quite thoroughly reviewed in the literature, these predictions generally start with some form of back-pressure, open-flow, drawdown or deliverability test in common use in the area in question. In most instances, a relationship is then established, or attempted, between this test and long-term performance through known or calculated producing characteristics of the reservoir. Also, in areas where the rate of gas sales is determined by regulatory bodies or contractual agreements, economic projections can often be based on fairly easily determined forecasts of stabilized rates of production throughout the majority of the producing life of the field.
However, in certain gas-producing areas there are no well testing requirements and uniform well tests do not exist except those that may be taken by an individual operator. The method described herein for the preparation of economic projections was developed in an effort to make possible such projections for gas wells in an area where maximum well capacity withdrawal is the general rule and where uniform testing procedures are not in evidence. The basis of the method is to calculate data for, and construct, a simple curve from production information gathered from numerous wells in the area and then project long-range well performance, on the basis of this curve, from a well's performance during its first 30 days "on the line".Finally, the possibility of relating long-term well performance to a short-term flow test is discussed.
Four groups of wells, consisting of 35 total wells, were considered in the study. Within the groups, the average daily production from each well for any given month on the line was related to the well's daily production during the month when its average daily production rate was the greatest. This month when the average daily rate of production was the greatest is called the "best month".