Abstract

This paper presents detail experimental and theoretical study and the results of successful applications of acid fracturing to enhance heavy oil recovery in Ordovician formation in LN region of TARIM, China. The Ordovician formation in LN field is extreme heterogeneous carbonate with naturally fractures. The reservoir temperature is 140°C, and the formation fluid has API gravity of 8.

To optimize the acid system for this complicated formation condition, we first studied the reaction characteristic of gelled acid compared with straight acids at temperature ranging from 60°C to 140°C. Acid conductivity experiments were conducted with a unique conductivity instrument using limestone samples designed to simulate alternating stages of pad acid fracs and closed acidizing process. The original way to determine acid/rock reaction rate at high temperature is to extrapolate from low temperature test data. Our test has shown that the actual reaction rate at high temperature is much less than that obtained from the low-temperature data. Also, results show that gelled acid is 2.5 times more retarded than straight acid and the delayed nature of gelled acids is useful forgenerating longer etched fractures. Gelled acids also have higher viscosity than straight acid, which reduces fluid loss and friction pressure in acid fracturing applications.

Based on the experimental result, some novel approaches were used to evaluate stimulation and improve heavy oil production. FMI logging technique was used pre- and post-stimulation, which shows that fracture numbers and shapes were much different from theoretical reorganization. Three kinds of fractures were observed, including vertical fractures, horizontal fractures and inclined fractures. A theoretical model considering wormholing was used to evaluate pressure decline after acid fracturing. In order to minimize post-stimulation production decline rate in field, a new technique, mixing light oil with heavy oil at bottomhole condition, was introduced into this area to significantly improved oil mobility.

Field cases of applying the new approach of acid fracturing in the Ordovician formation in TARIM over five years are presented. The field cases show the improvements in production of heavy oil resulting from the comprehensive approach.

Introduction

Acid fracturing is one of the effective stimulations of the oil-and-gas wells in carbonate reservoir. The production enhancement of a well after acid fracturing treatment is affected by the length of the etched fracture that remains open after the treatment and the conductivity of the fracture. The distance that reactive acid moves along a fracture is controlled by the rate of acid rock reaction, acid leakoff velocity, and acid convection along the fracture flow rate. It is necessary to measure reaction rate in order to determine the acid-etched fracture length. Acid reaction parameters with rock including reaction rate constant, reaction order, H+ diffusion coefficient and activation energy, are the major and basic parameters for design.

Many experts1 have done systematic researches on the acid rock reaction kinetics, however, there are few reports on acid rock reaction at 100°C and higher temperature. Instead, it was extrapolated from low temperature testing data. Roziers2 developped a new test instrument, a diaphragm diffusion cell, to obtain the reaction parameters at an experiment temperature of 63°C. The experiment manifested that the effective diffusion coefficient of gelled acid is one third of that of conventional acid. Mumallah3,4,5 proposed to obtain the acid rock reaction parameters and effective H+ diffusion coefficient with an annular flow meter at an experiment temperature below 93°C. Conway6 et al measured the H+ diffusion coefficients of conventional acid, gelled acid and emulsified acid with a diaphragm cell at an experiment temperature ranged from 21°C to 65°C. No reaction test was conducted at a high temperature in all the researches.

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