Abstract

According to tabulations on page 106 of the Atlas of Major Texas Oil Reservoirs published by the Texas Bureau of Economic Geology in 1983, there are 19 oil fields producing from the San Andres/Grayburg Carbonate on the south Central Basin Platform. These fields range in depth from 2200 to 4800 feet. The crude produced ranges from 28° to 37° API gravity. The reservoirs range in temperature from 80° to 119° (F) with the majority in the below 95° range. The expected percent recovery of the original oil in place ranges from 18% to 42% with the average given as 26%. In general the productive areas are limited by an updip permeability pinchout. Regional dip is gently easterly with the downdip reservoir limits being essentially the oil-water contact. Faulting probably is not a major consideration. The productive interval is in the range of 200 feet thick with small scale heterogeneity. Based on the depositional history of this area, local and larger scale variations in permeability and porosity both horizontally and vertically are expectable. Such variations undoubtedly have a marked effect on the overall percentage recovery. The problem then for all operators is how to economically obtain the greatest recovery from the remaining reserve. You all know and have evaluated the usual options. Many have been tried with varying degrees of success. Close infill drilling from mine workings, the most promising option, is usually discarded off hand with either of two main excuses. Either it can’t be done or it will be too expensive. Usually no effort is taken to determine whether either or both of these excuses is valid.

Structurally and stratigraphically this area is favorable for mine development. When one considers the current deepest shaft in the United States, the Hecla silver mine in the Couer de’ Alene area of Idaho is approximately 7700 feet deep, it is apparent the required depth would not be excessive. In all cases the rock temperature will permit the presence of mine crews although the one field at 119° (F) would be marginal.

Economics then is likely to be the governing factor. All operators should have good data as to probable mobile residual reserve within their holdings. They should be able to make a reasonable estimate of potential recovery expectable via infill drilling on one acre or closer spacing, and should know where the more prolific and the barren areas are likely to be. These data would be essential in placing a mine development. The data you do not have is how a mine-assisted access could help you, how it can be developed and what it would cost.

As would be expected, the mine shaft is the most expensive part of the development. Using modern systems of shaft sinking, a concrete lined round shaft 18 to 20 feet in diameter is about the cheapest. For this area, the shaft would likely be 24 or more feet in diameter. If possible, it should be located so as to pass through a barren and preferably dry part of the reservoir. It can be sunk through oil-bearing or water-bearing zones, but if that is necessary the cost per foot through those zones increases markedly. The shaft should be sunk to permit development of the mine workings in a hard, barren and hopefully dry stratigraphic unit 100 or so feet below the bottom of the oil bearing reservoir. For ventilation purposes and as a safety measure, a second shaft is required before extensive development.

It is suggested the mine system consist of a network of tunnels, much like the streets of a city, developed on 880 foot centers under the area expected to be productive. If driven with a machine called a roadheader, the tunnels would be approximately rectangular, probably 10 feet high and 15 feet wide. If driven with a tunnel boring machine, the tunnel would be round and 15 feet of more in diameter. In a good limestone, these machines should be able to develop 100 feet or more tunnel per day.

As soon as development has progressed into the area for well development, drilling of the production wells can begin. From then on mine development, well development, and oil production can continue simultaneously. Using the system and equipment described in U.S. Patents #4,458,945 and #4,595,239 it will be possible to drill into and complete wells in these reservoirs still under pressure. It is suggested wells be approximately 3 inches in diameter or larger if desired. They can be completed in any way recommended by the petroleum engineering staff. Well spacing or location can be wherever is recommended by the geological staff based on existing well data and control data that may be developed through mapping of the mine workings. The wells would be drilled with standard mining equipment operating through assemblies described in the above patents.

To aid in preliminary estimating of the economic potential of infill drilling from mine workings, the following figures can be used. Obviously a much more detailed analysis will be necessary before developing any particular area.