(Tech Connections Column, August 2006, American Oil and Gas Reporter)

Some of Oxford Oil Company’s Clinton Sandstone wells in Ohio underperform and are candidates for abandonment. Oxford shared the results of a four-well plug-back program at an Appalachian workshop.

The wells are located between two Berea Sandstone fields. Not having logs on its Clinton wells, Oxford looked to its Berea wells in the Crooked Tree Field to develop a recompletion strategy. In each case, the company plugged and abandoned the lower portion of the hole, and pulled, inspected and reran the recoverable casing. The wells were recompleted in two stages: the 30-foot lower Gordon interval first, followed by the upper Berea-Gantz interval. An economic analysis concluded that 240 million cubic feet of new reserves was added to the four wells.

In the same workshop, Alliance Petroleum Corporation reported on Upper Devonian multiple zone completions in Southeast Ohio. Alliance began by upgrading all its Upper Devonian sandstone maps to identify faults throughout the intervals of interest. Higher productivity had been associated with these faults. In older Berea wells, a pressure decline in Upper Devonian sandstones had been observed as the overlying Berea was produced.

New wells were drilled on air to total depth and a full suite of logs–including sonic and temperature–were run to identify reservoirs and gas shows. Alliance completed four sandstones in three stages. The “best” sandstone was always isolated, whereas two of the “poorest” sandstones were completed in one stage. The operator strove to keep the number of perforations to 20 or fewer per interval, and injection rates were held to 20 barrels a minute to stay in zone. Typically, 600 barrels per stage were pumped, but up to 900 barrels were pumped in the best zone or in a two-sandstone stage completion.

When located near faults, production was in an attractive 30-50 Mcf a day range. Away from faults, disappointing productivity of only 7-10 Mcf/d was realized. This confirmed that good sandstone is not enough; faults and fractures are required. Aeromagnetics and satellite imagery are being used to pick deep faults with a northeast-southwest azimuth that extend upward through these shallow zones of interest.

Sometimes results may be less than optimum because hydraulic fractures don’t go where anticipated. Universal Well Services shared the results from a three-well, multiple-zone Upper Devonian frac program on the Linden Hall lease. Fracture mapping had never been utilized in the Appalachian Basin, but in this case Pinnacle Technologies’ fracture mapping capabilities were employed to better understand what was happening.

Up to six sandstones in the Upper Devonian section are productive in the area, and are usually stimulated using a ball and baffle staging system. On average, all stages mapped grew along a NE-SW azimuth. However, fracture growth in cross sections was more complex. For example, if fractures from the first stage grew preferentially to the southwest, stage two fractures grew to the northeast as a result of a change in stress direction. Also, fractures were not confined to the individual sandstones in which they were initiated. Instead, they grew both up and down such that the fourth and fifth stages appeared to overlap each other. One conclusion was that the upper three zones could be perforated and stimulated in one stage rather than in three.

Elsewhere in a project supported by the state, Louisiana State University has been studying how the downhole water sink (DWS) concept applies to myriad Louisiana oil wells that are marginal or shut-in because of high water production. The state reportedly has 34,355 inactive wells. A great number of them are idle not because their resources have been depleted, but because high water cuts have made them uneconomic. Their many oil reservoirs are connected with strong aquifers that make them susceptible to coning, which leads to bypassed oil, which ranges from 40 to 90 percent of the oil in place with conventional completions. Bypassing oil is mainly promoted by high oil viscosity, large well spacing, high rates, small dip angles, etc.

DWS installations employ a dual-completion technique that co-produces essentially oil-free water from a lower set of perforations and much lower water-cut oil from perforations near the top of the oil leg. The DWS technology has potential to produce oil from wells that have entirely watered out. It also may produce oil from thin pay zones underlain by strong water columns. It has been applied in several international situations, but to date has not gotten much traction in the United States. The DWS approach may increase recovery by 15 percent. Granted, one may produce a lot more water (maybe three times as much), but since it is essentially free of oil, handling costs are much less.

These examples illustrate that although success is not guaranteed, one cannot afford not to try. Continuous learning is essential to success.