Abstract

Displacement of non-Newtonian waxy crude by cool and hot water is discussed with applications to the giant pilot test on hot water injection in Uzen field (Kazakhstan, ex-USSR). Analytical models for displacement of waxy crude by cool water, by hot water, by hot water slug have been derived taking into account paraffin precipitation, entrapment of oil by precipitated paraffins in blocked pores, heat losses and non-Newtonian reology of oil. It allows for simple graphic-analytical technique for prediction of the recovery factor. It was found out that paraffin separation affects the displacement at the latest stage of flooding only.

Introduction

Until 1986 ex-USSR used to lead the list of the world oil producers. Annual oil and condensate production in 1985 reached the level of 620 million tons. Ninety two percents of the total oil production was due to waterflooding There were a number of reasons why. Waterflooding completely dominated over the pressure depletion: successful implementation of the water flooding at the industrial scale during the second world war; high recovery was achieved in giant reservoirs by waterflooding in the late 50's; further improvements of waterflood technology were widely applied in giant reservoirs discovered in the 60's; later discoveries in the 70's were suited to the waterflooding as to the recovery method.

From the recoverable reserves in Russia, 2.6% come from waxy crude with paraffin contents up to 20%. Crystallisation and precipitation of paraffins result in decreasing of oil recovery.

Cold water flooding of high-paraffin oil reservoirs leads to a sharp drop in the oil production rate, rapid water encroachment of the producing wells and low sweep of the reservoir by the waterflood, due to the separation of solid paraffin in the pore space. The paraffin is entrapped by the porous medium, so that the pore capillaries are blocked, the permeability in single-phase flow falls sharply, and the oil recovery is reduced. It is therefore proposed to employ hot water injection in fields of this kind.

Two major waxy crude fields in the ex-USSR are Uzen (Western Kazakhstan) and Kharjagy (Russia, Timan-Pechora region). From 1966 and until now the largest in the world hot water injection is going on in Uzen.

In the current paper we develop a mathematical model for displacement of highly paraffinic oil by hot and cold water honouring: precipitation of paraffins and change of permeability and relative phase permeabilities, non-Newtonian properties of waxy cruds, variation of viscosity and effective viscosity with temperature, entrapment of oil by deposited paraffin particles in blocked pores.

A number of mathematical models for flow with paraffin deposition which honour more detailed schemes have been reported in the literature. G. Ali Mansoori discussed non-equilibrium kinetics of deposition. In large scale approximation under the well-to-well displacement, which is considered in the current paper, we assume equilibrium between paraffins in oleic solution and in solid state. Transport of paraffin particles inside oleic phase was taken into account to describe the process in the vicinity of the wellbore, where velocity is high. For displacement far away from wells inside the reservoir this effect can be ignored. We also do not consider the effect of gas phase below the bubble point pressure.

The models with deposition of paraffins contain a permeability reduction function. F. Civan proposed empirical correlation which was calibrated by laboratory tests.

In the current paper we propose a dynamic flow test in core which allows to restore the permeability reduction function by solution the inverse problem. The results are in a qualitative agreement with the data of percolation modelling.

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