Thief zones

Shelf (elastics) Sheet-like sandbodies resulting from storms or transgression. Usually thin but very continuous sands, well sorted and coarse between marine clays. Very high productivity but high quality sands may act as thief zones during water or gas injection. Action of sediment burrowing organisms may impact on reservoir quality. 

Microbial-enhanced oil recovery involves injection of carefully chosen microbes. Subsequent injection of a nutrient is sometimes employed to promote bacterial growth. Molasses is the nutrient of choice owing to its low (ca 100/t) cost. The main nutrient source for the microbes is often the cmde oil in the reservoir. A rapidly growing microbe population can reduce the permeabiHty of thief zones improving volumetric sweep efficiency. Microbes, particularly species of Clostridium and Bacillus, have also been used to produce surfactants, alcohols, solvents, and gases in situ (270). These chemicals improve waterflood oil displacement efficiency (see also Bioremediation (Supplement)). 

Microbes adsorb and grow on reservoir rock surfaces fed by injected nutrients (271) and may have appHcation in plugging thief zones near injection... 

Most squeeze cementing operations take place in cased sections of a well. However, open-hole packers can be used to carry out squeeze cement operations of thief zones during drilling operations. 

Oil recovery can also be affected by extreme variations in rock permeability, such as when high-permeability thief zones between injectors and producers allow most of the injected drive fluid to channel quickly to producers, leaving oil in other zones relatively unrecovered. A need exists for a low-cost fluid that can be injected into such thief zones (from either injectors or producers) to reduce fluid mobility, thus diverting pressure energy into displacing oil from adjacent lower-permeability zones. 

Volumetric sweep efficiency is determined by the permeability and wettability distribution in the reservoir and by the properties of injected fluids. Waterflooding characteristically exhibits poor volumetric sweep efficiency. The more expensive the injection fluid, the more important it is to have a high volumetric sweep efficiency so that the injected fluid contacts and thus mobilizes a larger volume of oil. High permeability streaks or layers (thief zones) and natural or induced rock fractures can channel the injected fluid through a small portion of the reservoir resulting in a low volumetric sweep efficiency. 

Crosslinked xanthan gums have also been used to reduce the permeability of thief zones. Trivalent chromium and aluminum have been used as crosslinkers (223,224). While crosslinker effectiveness is reduced at high salinity, Cr(III) has been used in the field at... 

The use of polyethylene glycol ethers in a process in which a high viscosity emulsion is formed on contact with residual crude oil has also been tested as a means of plugging thief zones using surfactants (248-250). Precipitation of sodium pectate when fresh water solutions contact brine has been proposed as a method of plugging high permeability zones (251). 

Even in the absence of fractures and thief zones, the volumetric sweep efficiency of injected fluids can be quite low. The poor volumetric sweep efficiency exhibited in waterfloods is related to the mobility ratio, M. This is defined as the mobility of the injected water in the highly flooded (watered-out) low oil saturation zone, m, divided by the mobility of the oil in oil-bearing portions of the reservoir, m, (253,254). The mobility ratio is related to the rock permeability to oil and injected water and to the viscosity of these fluids by the following formula ... 

An effective method to increase the efficiency of a fluid drive is to plug the thief zone(s), direct fluids to areas of higher oil saturation and thus improve the ratio of oil produced to fluid injected. Emulsions have been shown to be effective permeability modification agents, but little work has been reported on their use. 

At some point an area will develop where oil is preferentially removed perhaps because the porosity of the rock is greater, the oil less viscous, etc. In any case, the water flowing through this area will increase in volume and velocity further increasing the removal rate. Eventually, this area consumes much of the total water flow. Some recovery wells see much decreased water flow and therefore much less oil production. The area has become the path of least resistance to the water flow and because it steals most of the water flow, it is known in the industry as a thief zone. The development of such a thief zone is illustrated in Figure 12.74. Colloidal silica can be used to seal off such a zone and help restore higher production... 

Colloidal silica pre-treated to gel in a known time is injected and allowed to fill the thief zone. When the silica gels, the thief zone is plugged. 

No flow is seen here as the thief zone has been sealed... 

Oil production continues until another thief zone is generated and the sealing process is repeated... 

Profile modification is a process in which flooding water is diverted from highly permeable zones into less permeable oil-bearing layers within a subterranean reservoir. In recent years this has been achieved by the use of polymeric hydrogels in Y ich the water-soluble polymer is cross-linked by a multivalent metal ion". The latter may be generated in situ from a redox couple, e.g., sodium dichromate/sodium metabisulfite or as a complexed reagent, e.g., aluminum citrate. This enables the gelation process to be retarded until the aqueous polymer solution has been placed deep into the thief zone. 

R, H. Knapp and M. E, Welbourn, An Acryllc/Epoxy Emulsion Gel System for Formation Plugging Laboratory Development and Field Testing for Steam Thief Zone Plugging, Paper SPE 7083, Presented at the Fifth Symposium on Improved Methods for Oil Recovery of SPE of AIME, Tulsa, OK, April 16-19 (1978). 

In making laboratory measurements on adsorption, it is very helpful to consider which field cores should be selected and used for most prominent attention. For example, in a multilayer reservoir from which many rock samples are available, including low-permeability cores and higher permeability material, say from a thief zone, which should be selected for most experiments This issue will be addressed in the calculations presented later in this section. 

Borates have been proposed for use in different ways to reduce the permeability of zones in the reservoir in order to control the path or profile taken by injected fluids in water fiooding. In one approach, borates are used to form cross-linked polymer gels that reduce permeability and block off fluid access to thief zones. Another concept exploits the sharp reduction in borate salt solubility at low temperatures to cause a hot nearly saturated solution to form precipitates and block the rock pores, after being injected and on gradual cooling. 

Polymer front breakdown by fingering Polymer adsorption/ retention in porous media. Polymer degradation Thief zones Loss of mobiiity control and flood front Natural synthetic tracers. Polymer concentration in production water. Oil/water rates Multi-place measurement of viscosity and polymer concentration. Backflow tests. O2, Feand bacteria measurements On-site polymer viscosity tests before and after. Use of tracer and better defined geology. Wells inteqritv... 

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