Abstract

A new method to prevent sand permeation from unconsolidated or poorly consolidated reservoir formations has been developed. The Quasi Natural Consolidation (QNC)-method involves a controlled in situ precipitation of calcium carbonate scale on sand grains. Experiments show that calcium carbonate forms bridges between sand grains and strengthens the unconsolidated sand pack. The QNC-solution contains Ca2+, urea and urease. When this single-phase solution is injected into the sand pack, calcium carbonate precipitates at a rate which is dependent on the urease concentration and temperature. A series of batch experiments have been carried out in order to establish the optimum solution chemistry/composition for sand consolidation. Consolidation experiments have shown that this method is applicable in the temperature range from 25 °C to at least 65 °C. An untreated sand pack collapsed at a water flow velocity of < 0.01 cm/s (Q/Atot). However, after two QNC-injections at either 25, 50 or 60 °C the sand packs could withstand a water flow velocity >0.38 cm/s without producing sand. The permeability dropped about 25% from the initial ∼10 Darcy. Uniaxial strength tests showed that up to 10.56 MPa could be obtained after four injections. Consolidation was obtained also with presence of oil in the sand pack, although with a somewhat reduced efficacy of the treatment. Permeability measurements indicate that the relative water permeability is reduced more than the relative oil permeability after treatment.

Introduction

The aim of this work was to develop an inexpensive and environmentally friendly method to prevent sand permeation from unconsolidated or poorly consolidated reservoir formations. The problem regarding sand production during oil recovery is well known. When this occurs, the production must be reduced to avoid the problem. Local collapse of the reservoir may also occur with severe problems for the operation of the oil well. In addition, the presence of sand in the produced fluid raises production costs due to necessary removal of the sand before it creates problems. Several techniques have been applied for stabilization of sandy oil formations, such as, in situ chemical consolidation with various agents.[1–3] Mechanical steel screens and gravel packs between the sandy formation and the well are used frequently.[4] These techniques give good results. However, the chemical methods used today have serious drawbacks due to the expensive and toxic chemicals. The sand screen and gravel pack installations are very expensive. Sand consolidation by precipitation of the inexpensive and sparingly soluble salt calcium phosphate has been presented in a previous work.[5] However, the complicated injection/mixing procedure needed for a good consolidation made this consolidation method prohibitive for application in real reservoirs.

In this work consolidation is obtained with calcium carbonate, CaCO3, precipitation. CaCO3 comprises approximately 4% of the earth's crust6 and because of its importance in geological and biological environments, and as a scale problem during oil production the precipitation of CaCO3 from supersaturated solutions has been extensively investigated. However, enzymatically induced calcium carbonate precipitation has not received that much attention. Enzyme-based production of minerals such as calcium carbonate and calcium phosphate for sand stabilization was reported by Harris, et al.[7,8] However, very little data on consolidation and kinetics of precipitation were reported.

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