Field Case: Match of a 600 Bars Depletion and 40 Years of History in a Fractured Carbonate Sour Gas Field
Authors: Paux F. , Zhou H. 
Published: December 2007

Abstract
The Lacq field, in South - West France, produces from deep, fractured, carbonate reservoirs. Formation fluid is sour gas (15% H2S - 10% CO2). Production began in 1957; static pressure was 660 bars and temperature 130 C at -3700 vm/SL. Static pressure was 57 bars in 1995.
This paper presents the results of a study performed to match the production history and optimise the final stages of production. Unconventional phenomena were evidenced and introduced in the reservoir simulator:Evaporation and degassing of connate water.Increase of permeability with depletion.Dual porosity representation.
Their influence proved essential to understand the dynamical behaviour of the field over the large depletion interval and achieve an adequate pressure (less than 1 bar) and composition match.

Introduction
With an OGIP of 260.10 9 Sm3 (9181.10 9 SCF) the Lacq field can be considered a giant. Gas pay is over 2000 meters (6561 ft), which implies large pressure and temperature variations (130–175 C / 266–347 F) in the reservoir, responsible for complex thermodynamical phenomena. The reservoir is made up of tight carbonates. Production is only possible due to dense fracturing which increases with the depletion. Initial pressure was 660 bars at 3700 vm/SL (9750 psia at 12139 ftss), which means that the reservoir was over-pressured. The large depletion (600 bars/8700 psi) due to the 40 years of production triggered unconventional phenomena, usually negligible and unnoticed on more conventional fields.
Reservoir studies undertaken since 1994 allowed characterization of these unusual phenomena. Modifications were made to the reservoir simulator to accommodate them and, after a phase of history match, to provide production forecasts and estimates of remaining reserves.
Objectives of a study on a highly depleted field
During the first 18 years of its producing life, the Lacq field had the behavior of a classical gas reservoir with no aquifer. Production forecasts were easily made using a standard plot of P/Z versus cumulative gas production (Fig. 1).
In 1975, pressure measurements started to show a drift from the initial slope on the P/Z plot, apparently indicating a decrease in reserves. An explanation was found with the startup a year earlier of disposal water injection in the periphery of the field: the imbibition of this injection water was trapping gas in the matrix blocks. A reduction of the injection rate was implemented to maintain the loss of reserves to an acceptable level.
However, in 1986, the P/Z line started drifting in the opposite direction. This phenomenon was noticed shortly after a change in the plateau rate from 21.106 Sm3/d to 8.106 Sm3/d (743 to 283.106 SCF/d). It was believed that peripheral tight zones were triggered by the decrease in withdrawal rate.
By that time, the P/Z approach had already shown its limits and a new model had to be built to evaluate the remaining reserves and corresponding investments.
The last event happened in 1988 when, after 30 years without change, the H2S content in the gas started to increase steadily. It seemed obvious that many unexpected phenomena had been triggered by the high depletion (over 600 bars). Reservoir fluid behavior
The reservoir fluid composition was determined by Perraud & Lazarre in 1963:
At reservoir temperature the two phase region is not reached during the depletion. The fluid is wet gas.
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