Case History of the Meillon Gas Field
Authors: Golaz Philippe , Sitbon A.J.A. , Delisle J.G. 
Published: December 2007

Summary
This paper presents results of studies performed to determine how waterinvasion in tight, fractured carbonate formations governs gas productivity. Seismic and dipmeter data were used to create a new structural picture of theMeillon field. Although the wells located at the top of the structure areinvaded by water, we concluded that the field was not totally invaded, but waslocally invaded. As a result, new wells are being drilled, compressors arebeing installed to increase production, and water is pumped out. production, and water is pumped out.

Introduction
Meillon gas field was discovered in 1965 with the Meillon 1. It is locatedin southwestern France near the Lacq gas field. The east/west reservoir is 17miles [27 km] long, 2 miles [3 km] wide, and 14,760 ft [4500 m] subsea. Part ofit is below the town of Pau. Original sour gas in place was evaluated to beappx. 2,650 × 109 scf [appx. 75 × 109 std m].
Many wells were drilled in the area, but acquiring a good understanding ofthe reservoir's structural features was difficult because it is highlydislocated.
For 10 years gas production was waterfree; it was thought that the reservoircould be depleted with the 10 existing wells (two clusters) located at the topof the structure near the fault. In 1978, cumulative production was 918 × 109scf [26 × 109 std m] production was 918 × 109 scf [26 × 109 std m] when wellslocated 2,300 ft [700 m] above the initial gas/water contact (GWC) started toproduce water. Five new wells were drilled along the structure between 1978 and1986 as older wells stopped producing because of water-lift problems. Actualcumulative production was 1,520 × 109 scf [43 × 109 std m].
A structural study performed to understand the water entry showed that openfractures are oriented in an east/west direction, especially along the southernfaulted area. This finding completely changed our understanding of the dynamicreservoir behavior.
A numerical simulator that accounted for a permeability contrast in theeast/west and north/south directions was used to match the production historyduring the water-free production history during the water-free period. period. This paper describes the hypotheses derived from these observations and thepractical actions implemented. practical actions implemented. FieldDescription
Structure and Geology. Field development was impaired by stratigraphiccomplexity and difficulty in seismic interpretation. The reservoir, located inthe Jurassic formation, is an anticline structure with three main communicatingareas, Baysere and Pont d'As, St. Faust, and Mazeres (Fig. 1). The northernflank dips 20 to 30 deg. north; the southern flank is reduced by anunconformity and a major east/west fault (Fig. 2). The reservoir is 660 to 820ft [200 to 250 m] thick, and the initial GWC is located at 16,275 ft [4960 m]subsea. (This was deduced from pressure measurements in Wells Lons 1 in theaquifer and Meillon 1 in the gas phase.)

Reservoir Characteristics.
The reservoir is a low-porosity (vugular in someareas), low-permeability dolomite, with a porosity ranging from 1 to 8%, and amatrix permeability less than 1 md. Two to three fractured zones are randomlylocated over each well.

Reservoir Contents.
The reservoir contains sour gas underlain by water. Thegas column reaches a maximum of 2,800 ft [850 m] at the crest of the structure,13,450 ft [4100 m] subsea. Table 1 presents the gas composition. Initialpressure at 13,450 ft [4100 m] is 6,960 psi [48 MPa], and temperature is 300deg. F [150-C].

Volumetric Estimate of Gas in Place.
The volumetric evaluations are fairlyinaccurate because of the important uncertainties in all parameters (e.g., structural and petrophysical parameters (e.g., structural and petrophysicaldata). Initial gas-in-place estimates ranged from 2.3 × 1012 to 3.53 × 1012 scf[65 × 109 to 100 × 109 std m].

Field Development and History
Field Development. Gas production started in Oct. 1968, with three wells(Baysere 1, Pont d'As 1, and Pont d'As 2) located at the Pont d'As 1, and Pontd'As 2) located at the west end of the structure. Some wells (Pont d'As 3 and4; St. Faust 2 through 5, 7, and 2; and Baysere 2) were drilled eastward in thewestern half of the reservoir until the end of 1971.
All wells were located on the crest of the structure, and the field wasinitially developed with two well clusters. Some wells started producing waterin 1978, and field productivity declined dramatically (Fig. 3). productivitydeclined dramatically (Fig. 3). The eastern part of the reservoir becameisolated, and the pressure profile from west to east showed a pressure sinkthat was created in the center of the reservoir, where well concentration washighest (Fig. 1).
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