This paper describes a new method for water control by the use of bullhead injection. A water based gelant is emulsified in oil and injected into the formation. The emulsion is designed to separate into a water phase and an oil phase at static conditions in the formation. Upon reaction in the formation the water phase gels up while the oil phase remains mobile. It has been found that the controlling parameter for disproportionate permeability reduction (DPR) is to control the fraction of gel occupying the porous media. The water fraction in the emulsion controls the reduction in relative oil and water permeabilities. A program was undertaken to verify this DPR method in a field test, using a commercial blocking gel system. The first treatment was performed in well 30/3 A 16 T2 at the Statoil operated Veslefrikk field offshore Norway. Results show that water production was reduced by 30% after the pilot test, while maintaining the oil rate. As expected, total well productivity was reduced by more than 80%. The treatment consisted of 124 m3 emulsion, bullheaded from surface. Step rate testing and ion water analysis were combined to study the relative change in flow contribution between the 6 perforated intervals. Introduction The increasing number of high water-cut producing wells requires simple and cost effective water control technologies. One option is bullhead injection of chemicals capable of reducing the water permeability more than the oil permeability, normally described as Disproportionate Permeability Reduction (DPR) or Relative Permeability Modification (RPM). DPR will be effective in multilayered reservoirs without crossflow and with some zones producing clean oil or in treating coning problems.[1–3] In such situations DPR treatment will reduce the water cut and may result in increased oil production. The existence of DPR fluids is well known.[4–8] DPR fluids may be classified as polymer systems, weakly crosslinked gel systems or rigid gel systems. The polymer systems are the most frequently used. The DPR mechanism has been explained by polymer adsorption on a water-wet rock surface.[9,10] This will reduce the effective pore space for flowing water. The ratio between the thickness of the adsorbed layer, e, and the pore radius, r, is used to express the permeability reduction, RRF (defined as the ratio between permeability before and after treatment), using Poiseuille flow in a bundle of capillary tubes. ………………………………………..(1) The oil will flow more or less un-restricted in the middle of the pores. Consequently the oil permeability reduction, RRFo, will be lower than the water permeability reduction, RRFw. From Eq. 1 the following can easily be derived:RRF will decrease with increasing pore radius, i.e. increasing the permeability.RRF will increase by increasing the thickness of the adsorbed layer. The latter one can be obtained by using polymers with larger molecular weight and higher affinity for adsorption. Alternatively weak gels or aggregates can be formed in-situ by injection of polymer and crosslinker.[5,11–12] Often, the polymer and the weakly crosslinked gel systems involve additional retention mechanisms, such as pore trapping. The use of preformed gel particles will also increase the effective adsorption thickness.[13] Rigid crosslinked gel is normally used for total blocking. Blocking gels have DPR properties[7,14–17] but for practical purposes in matrix treatment the oil permeability reduction has been recognized far too high for bullheading. However, crosslinked gels could be promising alternatives if the permeability reduction, RRF, can be controlled. This is mainly because crosslinked gels show (i) better temperature stability, (ii) DPR properties regardless of wettability and (iii) high RRF also in high permeability zones. A crosslinked gel will probably also be more resistant towards the high shear rates close to the wellbore. Consequently the volume of active DPR fluid can be reduced. It has been demonstrated that RRF depends on the fraction of gel occupying the pore space.[15–16,18]

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Veslefrikk is a mature oil field in the Norwegian sector of the North Sea. It has a layered structure with several pressure-independent layers and three main fluid systems: the Statfjord Formation, Intra Dunlin Group and Brent Group. Each fluid system has distinct formation water compositions but similar oil compositions. Seawater injection has been used for pressure support and many wells have commingled production. The principal scale control challenges are scaling potential and placement of scale inhibitor into zones with different pressure. Although production logging is performed regularly to aid scale squeeze designs and production allocation, this is costly, involves well intervention risks and it cannot always be undertaken when required. In this study an innovative approach to obtain ‘production logging type’ information from wells producing from two zones has been evaluated. It involves undertaking multi-rate well tests where produced water samples are collected at each rate. Depending on the conditions for each test, either qualitative or quantitative integrated analysis of the well test results and produced water compositions can be performed. Information obtained from the tests for each zone includes: Pressure, productivity index, water cut, produced water composition and seawater fraction. The methodologies adopted for the multi-rate tests and the analysis of their results are described, and the results for four multi-rate tests are presented. The multi-rate tests were undertaken under different conditions and the influence of these conditions on the type and quality of results obtained are discussed, and assumptions and associated uncertainties are identified. The case studies demonstrate that information useful to scale, well and reservoir management can be obtained from multirate tests. In addition, produced water compositions can be obtained from these tests without the need for downhole sampling. However, in each case it is important to assess uncertainties associated with the results.

Veslefrikk is a mature oil field in the Norwegian sector of the North Sea. It has a layered structure with several pressureindependent layers and three main fluid systems: the Statfjord Formation, Intra Dunlin Group and Brent Group. Each fluid system has distinct formation water compositions but similar oil compositions. Seawater injection has been used for pressure support and many wells have commingled production. The principal scale control challenges are scaling potential and placement of scale inhibitor into zones with different pressure. Although production logging is performed regularly to aid scale squeeze designs and production allocation, this is costly, involves well intervention risks and it cannot always be undertaken when required.In this study an innovative approach to obtain 'production logging type' information from wells producing from two zones has been evaluated. It involves undertaking multi-rate well tests where produced water samples are collected at each rate. Depending on the conditions for each test, either qualitative or quantitative integrated analysis of the well test results and produced water compositions can be performed. Information obtained from the tests for each zone includes: Pressure, productivity index, water cut, produced water composition and seawater fraction.The methodologies adopted for the multi-rate tests and the analysis of their results are described, and the results for four multi-rate tests are presented. The multi-rate tests were undertaken under different conditions and the influence of these conditions on the type and quality of results obtained are discussed, and assumptions and associated uncertainties are identified.The case studies demonstrate that information useful to scale, well and reservoir management can be obtained from multirate tests. In addition, produced water compositions can be obtained from these tests without the need for downhole sampling. However, in each case it is important to assess uncertainties associated with the results.