Two basic factors which are especially prevalent in typical producing oil and gas wells in the Gulf of Mexico and the surrounding coastal regions have created the need for a new plugging technique. First, the majority of oil and gas wells in the Gulf Coast region produce from reservoirs which are commonly classified as water-drive type reservoirs. In a water-drive reservoir, the predominant mechanism which forces the movement of oil or gas in the reservoir toward the wellbore is the advancement of a formation water aquifer. The formation water phase is found beneath the oil or gas phase in a "bottom-water" reservoir or on the outer flanks of the oil or gas column in an "edge-water" reservoir. In either case, water moves into the rock pore spaces which were once filled with hydrocarbon fluids in response to continued production of oil or gas. Over time, this natural water encroachment leads to the advancement of water into the producing interval, and the well eventually begins to produce quantities of formation water. As the influx of water continues in the reservoir, the percentage of produced water, as compared to total fluid production, increases with time.
The ever increasing production rate of formation water is undesirable in both oil and gas wells. In the case of an oil well, the energy required and correspondingly the cost required to artificially lift a given volume of oil from a well must be proportionately increased if formation water is being produced together with the oil. Therefore, the reduction or elimination of water production from an oil well is economically advantageous since: (a) lifting costs to produce the oil are reduced, and (b) costs associated with the treatment and proper disposal of the produced waste water are lowered.
In the case of a gas well, the production of even relatively low quantities of formation water can be detrimental to the productivity of the well. When formation water and natural gas enter the wellbore, each fluid phase begins to travel upward toward an environment of reduced pressure at the surface of the well. As pressure decreases toward the surface, gas contained in the well's tubulars expands, and the velocity of the gas increases accordingly. As a result, the expanding gas acts as a carrying mechanism to continually remove the formation water from the well. However, as reservoir pressure decreases in response to continued gas production and/or water volumes entering the wellbore continue to increase, the ability of the gas to carry and remove formation water from the well is greatly reduced. As this phenomenon begins to occur, the relatively dense formation water begins to "fall back" into the well. Eventually this water will fill the well's tubing to the point that the hydrostatic pressure created by the water column approaches the prevailing reservoir pressure, and the productivity of the well is significantly reduced. Increasing water encroachment and/or continued pressure declination results in the eventual cessation of production.
The second basic factor, which is typical of Gulf Coast oil and gas production, is the common occurrence of unconsolidated sandstone reservoir rock formations. In this type formation, sand grains (which make up the sandstone rock) do not contain adequate intergranular cementation or rock strength to ensure rock stability during the production of oil and gas. As a result, the rock, in its natural state, often fails when subjected to the stresses imposed on it during the production mode. Small rock fragments are then produced into the wellbore. Once accumulated in the wellbore or well tubulars, this fine grain material possesses a permeability that approaches zero, and well productivity is greatly reduced.
Various techniques to increase the stability of the sandstone reservoir rock (or methods of "sand control") have been employed through the years. One common method of sand control which was employed extensively during the nineteen sixties and early seventies was the pumping of a compound through the formation pore spaces that, once cured, would coat the sand grains and add "artificial" grain-to-grain cementation. Thus, the overall rock strength was increased in the treatment area. This method of sand control had a positive effect on dealing with the previously mentioned problem of natural water encroachment into a well's producing interval. The completion interval utilizing this method of sand control remains essentially free of downhole mechanical equipment. Therefore, at the point in time when water moves into the completion interval, an attempt can easily be made to "plug-back" the lower portion of the well. This is accomplished by lowering, on electrical wireline equipment, an expandable plug through the well's production tubing and into the cased region of the well. The plug is positioned immediately beneath the completion interval, and the plug is expanded to contact and affix itself to the casing wall. Once in place, the plug acts as a "bottom" such that commonly used oilfield cement can be placed on top of the plug to the desired height in the well. The placement of cement up to or above the oil- or gas-water contact in the reservoir, if successful, has a limiting effect on water production.
With the advancement of alternate sand control techniques, there came into use a new and preferred method of sand control commonly referred to as "gravel packing." The widespread use of this technique occurred in the nineteen seventies and is presently the sand control method of choice in most areas of the world. Gravel packing is a system in which uniformly sized and shaped sand grains are placed in a well's perforations and in the annular volume between the well's production casing and a slotted or perforated pipe, which is externally wrapped with wire. The sand grains, or "gravel," are slightly larger than the formation sand particles, and since they are tightly "packed" together, they prevent the collapse of the perforation tunnels and act as a filter to prevent the migration of formation sand into the wellbore. The wire wrapped or slotted "screen" is normally placed between two packers and contains the sand in an area adjacent to the perforated interval and prevents the movement of the gravel into the production tubing.
In the event that formation water moves into the completion interval, the "plug-back" method previously described is ineffective in reducing water production. The reason for this is that particulates in the common oilfield-type cement "plate out" and bridge off at the gravel pack sand face. Only the liquid (usually water) filtrate of the cement effectively permeates the gravel pack sand, and the cement dehydrates and cures as a plug inside the screen and slotted or perforated blank pipe. Even if the cement plug is spotted properly in the water-bearing portion of the producing interval, the formation water will simply flow vertically upward in the annular gravel-packed area surrounding the plug; and, if this distance is not sufficient in length, the cement plug will have very little effect on reducing water production.
For similar reasons, the placement of a mechanical plug inside the slotted or perforated blank pipe yields similar results. In this case, formation water enters the wellbore below the plug (which is set at a depth just above the oil- or gas-water contact) and travels "around" the plug, utilizing the high flow capacity annular area between slotted or perforated pipe and the screen. The limited effect of both of the above-mentioned methods to reduce water production from the lower portion of a gravel-packed well has led to the method of this invention.