East Texas Oil Field

Woodbine Field
Saltwater Disposal
Operational Trends
Lower Pay Zones

M-C Production and Drilling Co., Inc. Woodbine Field
East Texas (Woodbine) Field located in Gregg, Rusk, Smith, and Upshur Counties
The following information is provided concerning the East Texas oil field. The field has two outstanding features: it's tremendous size, and the simple nature of the geologic trap. The East Texas oil field covers over 132,000 productive acres, and at one time had over 26,000 actively producing wells. Since it's discovery in 1930, the field has produced over 5,110,339,000 barrels of oil and over 501 billion cubic feet of natural gas. The field produced 33,512,688 barrels of oil in 1991, and is expected to produce about 460,000,000 more barrels before being depleted. There are currently about 11,400 active wells, with about 8,000 of those producing.In a generalized summary, the oil in the Woodbine reservoirs within the field is trapped where the eroded, up-dip edge of the sand section crosses a regional structure nosing on the west flank of the Sabine Uplift. The sands are overlain by dense Austin Chalk, forming an impermeable seal above, and underlain by dense Washita Limestone which seals from below. The more massive Upper Woodbine sand reservoir may be properly described in this way. However, the Lower Woodbine sands are not as continuous across the field, and thus some localized, stratigraphic trapping occurs. Within the field, the Woodbine sands become progressively eroded from top to bottom in an eastward direction. The western edge of the productive area is defined by the original oil-water contact of -3320 with respect to the Woodbine reservoir.The oil is produced at an average depth of 3700'. The field has a water drive reservoir mechanism. The active water drive is not capable of keeping up with the large volume of fluid being produced, so the saltwater is re-injected into the down-dip edge of the field. This helps to maintain reservoir pressure, dispose of oil field brine, and increase ultimate oil recovery. As water encroachment occurs in various parts of the field, individual wells may require remedial work to delay watering out of the wellbore. This can be accomplished by plug-back procedures, thus producing from higher within the oil column.
Saltwater Disposal
Salt Water Disposal and Injection
Early in the productive history of the East Texas oil field, it was recognized that saltwater disposal and reservoir pressure maintenance were needed to ensure the highest ultimate recovery of oil. Since there is a low gas content in solution in East Texas oil, almost every barrel of crude produced from the Woodbine sands must be replaced by a barrel of water if the reservoir pressure is to be maintained. Some natural water encroachment is present from west to east within the field. The production of a barrel of saltwater has practically the same effect on the bottom hole pressure as the withdraw of a barrel of oil. Apparently, at higher rates of production, the sand is not capable of supplying sufficient water to replace the oil and water that is removed from the reservoir. If the produced saltwater is returned to the reservoir, the effect on the reservoir pressure is almost the same as if the saltwater had not been produced.The first saltwater was returned to the reservoir in June of 1938. The Texas Railroad Commission, determined in 1942 that the injection of saltwater produced from the field back into the reservoir was a conservation practice and would tend to decline the arrest the decline in pressure and would increase the ultimate recovery of oil. In order to encourage saltwater injection, the Oil &Gas Division of the Railroad Commission, by Order No. 6-78,651, effective July 1, 1983, allowed one barrel of additional oil to be produced for every 240 barrels of saltwater returned to the reservoir, with no more than 1.25 barrels of oil allowed with respect to any one well. Only Woodbine water produced with the oil is injected into the reservoir.There is some injection on the part of the operators in the field, but for the most part, the injection is carried out by the East Texas saltwater Disposal Company. This company was organized in 1942 to carry out injection on a field-wide basis for all operators. It's objective is to maintain close cooperation between the state and the operators in the field with respect to the program of saltwater injection and production control to effectively maintain the reservoir pressure and thus extend the life of the field and increase the ultimate recovery of oil from the field.
Operational Trends
Current Field-Wide Operational Trends

As producing properties within the East Texas Field continue to be sold and purchased, certain categories of operations have become apparent. Each of these operational trends is related to the nature of the field and the specific characteristics of the Woodbine reservoir. These categories are summarized as followed:1.Producing leases may be in repairs to surface equipment to ensure the most effective production of the oil and associated re-injected saltwater. Examples would be repairs to pumping units, flow lines, tank batteries and separators.2.Individual wells may be in need of repairs to downhole equipment to ensure the most effective production of the oil and associated re-injected saltwater. Examples would be repairs to rods , tubing and downhole pumps.3.Individual wells may be in need of plug-back to shallower pay zones within the Woodbine to decrease water production and increase oil production. Examples for a cased hole completion would be squeezing off existing perforations and re-perforating uphole. Examples for an open hole completion would be plug-back of the lower hole with gravel and plastic, thus raising the producing level uphole.4.Existing wells may be in need of deepening to explore for undrained pay sands. Examples would be wells that were originally not drilled to a sufficient depth to penetrate the entire productive Woodbine section present above the original oil-water contact.5.Existing leases may be in need of additional wells to properly drain all remaining oil reserves. Examples would be the drilling of new wells to compete with recent successful drilling or re-completion activity from offset operators in new pay zones. Another example would be the drilling of a replacement well for an existing well with unrepairable production problems.6.In some cases, wells with existing production allowables may develop unrepairable production problems such as casing leaks. The production allowable from such wells can be transferred to other wells that are capable of producing at rates higher than their current allowable. These casing leak wells can be plugged and abandoned, but the owner of the well is allowed to share the production from the receiving wells. This is known as a casing leak allowable transfer, and the wells are a special category CLAT wells so long as there is support from offset producing wells.
Lower Pay Zones
The Potiential for Lower Woodbine Pay Zones
The Woodbine sands are currently being re-explored for untouched stratigraphic oil traps within the boundaries of the giant East Texas field. This field, located in the central portion of the East Texas basin, covers over 130,000 acres. It is over 35 miles long and, in places, over five miles wide. The field has produced over 5.1 billion barrels of oil since it's discovery in 1930. Annual production in 1991 was 33,512,688 barrels of oil, and production in 1992 is estimated to have been over 30,000,000 barrels. Yet large areas within the field remain untested for deeper pay sands.Recent deeper well control, both within the field and around the outside edges of the field, has provided important information about the deeper pays in the lower Woodbine section. Numerous operators, including both major oil companies and smaller independents, are currently re-evaluation the current Woodbine potential by drilling new, deeper tests and by deepening existing wells that originally did not penetrate the entire productive Woodbine section present above the original oil-water contact. These efforts have succeeded in finding many new Woodbine pay sands in the lower part of the section. Study of this new well information indicates a specific trend to these discoveries and thus offers an opportunity to explore for profile Woodbine oil pay sands in a lower-risk setting.The East Texas Woodbine field is a north-south trending stratigraphic trap formed by an up-dip (eastern) erosional pinch-out of the Woodbine sands. The structure on the top of this unconformity dips to the west, with the original down-dip limit of the field being the -3320 oil-water contact. The up-dip limit of the trap in places is -3100, giving over 220' of original oil column. The thickness of the overall Woodbine section varies from 0' on the up-dip eastern edge to over 200' on the down-dip western edge. The Woodbine section in the field area is overlain by the Austin Chalk, a fine-grained, non-porous limestone that provides an excellent trapping cap to the underlying porous sands. The Woodbine is underlain with the Buda-Georgetown limestone section.Study of the well information indicates that the up-dip edge of the field, approximately from 0' to 50' of Woodbine section present, has been heavily drilled and has thus found most of the productive lower Woodbine sands in that area. Study also indicates that in order for lower Woodbine sands to be productive, they must present in a structural position up-dip to the original oil-water level at -3320. The contact may vary slightly from well to well, but this figure is a reliable estimate. When the overall Woodbine section is thicker than 150', most lower sands are below the original oil-water contact and are not potentially productive. Exceptions may exist to this general rule, but they are rare.Thus a trend is established, between the 50' and 150' values for overall thickness of the Woodbine section, where lower Woodbine sands can be found to be potentially productive. Most of the original wells in the field, drilled in the 1930's and 1940's, penetrated 50' or less of the Woodbine section. Typically, the wells were drilled approximately to the base of the Austin Chalk, then casing was set. The wells were then drilled out into the upper portion of the Woodbine section present and open-hole completed. It was rare for a well in this “50' to 150'” to penetrate the entire Woodbine section.The recent activity within the field has confirmed that this trend is the place to be when trying to locate new, lower Woodbine pays. Pay stands in this lower section vary from 2' to 202' in thickness and can cover relatively large areas. Regional deposition of Woodbine sands occurred from northwest to southeast. However, individual lower sands may vary greatly as to their strike direction, depending on whether they have been reworked and re-deposited prior to upper Woodbine sand deposition. Thus, mapping of each individual sand is required for proper development.Reserve potential for individual wells varies with the thickness of the sand and the overall size of the reservoir. Average porosity is 26% and permeability can exceed 4,000 millidarseys. The oil gravity is 39 degrees API. Produced saltwater is handled by a field-wide disposal system and is re-injected into the reservoir on the down-dip edge of the field. Using a recovery of 1,210.22 barrels per acre-foot, examples of reserve potential for an individual well range from 30,255 barrels of oil for a 5' sand draining 5 acres, up to 242,043 barrels of oil for a 20' sand draining 10 acres.
M-C Production