Abstract

This paper describes laboratory and modeling work to utilize high-sulfate pit brine and calcium chloride solutions as a means to the permeability of water producing zones. The study included compatibility tests, coreflood experiments, and scale prediction to calculate SI and SA at surface and reservoir conditions. The pit brine contains sulfate ions at 120,250 mg/L and its TDS is nearly 400,000 mg/L. Three solutions that contained calcium chloride at 10, 20, and 30 wt% were prepared. Coreflood tests were conducted using carbonate core of 100 to 1,100 md. Tests were conduced at 200°F (reservoir temperature) and overburden pressure of 2,000 psi. Most coreflood tests were conducted at 1 cm3/min. The pressure drop across the cores was monitored during these tests.

Compatibility tests indicated instantaneous precipitation of calcium sulfate when pit water and calcium chloride brines are mixed. Scale predictions are done for 10, 20 and 30 wt% CaCl2 brines with pit brine. The maximum super-saturation at 200oF for CaSO4 occurs at 45, 60 and 70 vol% of pit brine for 10, 20 and 30 wt% of CaCl2, respectively. Predicted maximum anhydrite is nearly 55, 81 and 97 g/L for the three CaCl2 brines, respectively. Coreflood tests indicated that maximum core plugging occurs when pit brines is injected into cores saturated with CaCl2 brines. The degree of core damage depends on the injection rate. Core flood results were explained in terms of thermodynamics and viscous fingering effects, which occur when a less viscous fluid displaces a more viscous fluid in porous media.

Water production is a serious concern for oil and gas companies. On one hand, there is a cost required to lift this water, and to perform separation, treatment, and then disposal. In addition, water production causes scale, corrosion and emulsion problems. Therefore, every effort should be made to minimize excessive water production. Current techniques that rely on chemicals (cross-linked polymers) are expensive and in some cases these chemicals are not environmentally friendly. The technique described in this paper is simple, and utilizes simple and green chemistry. It can be used to plug water-producing zones in carbonate and sandstone reservoirs.

This paper introduces a simple, effective technique that can be used to reduce the flow into high permeability zones. This technique can be used to minimize drill string sticking that occurs during drilling and reduce the amount of produced water during production Coreflood tests indicated that this technique is very effective in reducing the permeability of reservoir cores.

Introduction

Excessive water production causes major economic problems in terms of loss of oil production, as well as lifting, separating and disposing of large amounts of wastewaters. Produced water can be reduced or avoided by using mechanical or chemical means. Mechanical means include drilling horizontal wells, placing a liner then perforating the target zone, or using downhole separation equipment, e.g., a hydrocyclone.

Selection of a specific water shut-off treatment depends on the sources of water, which include: coning, casing leaks, high permeability streaks, and natural fractures. Understanding reservoir characteristics and water movement in the reservoir are key factors that determine the success of these treatments.

Another problem that is caused by the presence of high permeability zones is differential pressure sticking, which occurs during drilling across porous and permeable formations. Differential pressure sticking occurs when the drill pipe becomes stuck in the filter cake on a permeable formation during drilling. As illustrated in Fig. 1, during drilling, mud and filtrate are lost into permeable zones when the mud hydrostatic pressure in the wellbore, Ph, is greater than the formation pore pressure, P. As the filtrate is lost, the solids in the mud are deposited on the formation face as filter cake. The characteristics of the filter cake depend on the type and amount of solids present in the mud. A mud with high native solids content leaves a thick sticky filter cake, while a mud with bentonite produces a slick thin filter cake.

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