7,400
7,500 Zones
7,600 1
7,700 2
7,800 3
7,900
8,000
7,000 8,000
Water production
4,000 5,000 6,000
3,000 2,000 1,000 0
Oil production
400 500 600
300 Wellhead pressure
10/29/01 11/28/01 12/28/01 1/27/02 2/26/02 3/28/02 Date
4/27/02 5/27/02 6/26/02 7/26/02
100 200
0
700 800
> Galley eld well G5 log and production history. The horizontal section of this well passes through a channel, then a shale section and three productive zones (top). The shale section from 7,440- to 7,570-ft [2,267- to 2,307-m] measured depth (MD) collapsed during completion operations. Oil production declined steadily starting in January 2002, with water production increasing beginning in April 2002 (left). This well later had a matrix acidizing treatment.
the Heidrun A-48 well provided vital information to shorten the learning curve for treatment of other wells in this complex, clay-rich formation.
Damage Mechanisms in the Galley Field Operator ChevronTexaco used the new acid- stimulation process in the Galley fi eld on the UK continental shelf. The G5 well was completed horizontally with a 650-ft [200-m] openhole section in the late Paleocene-age Cromarty formation, which comprises
fine to very
fine-grained, poorly consolidated, turbiditic sandstone (above). Most of the productive sec- tion has a 100-mm mesh screen in place that was originally intended for a gravel pack, but a shale section about a quarter of the way along the horizontal section collapsed. Although pro- ductive sand channels beyond the collapsed shale are accessible for flow into the wellbore, those sections could not be gravel packed.19 Oil production declined steadily from an ini- tial 7,000 B/D [1,100 m3/d], but the oil decline rate accelerated when water production increased in April 2002. Before the stimulation treatment, the well produced about 1.1 million bbl [175,000 m3] of oil and 979 MMcf [28 million m3] of gas, along with about 31,000 bbl [4,900m3]
of water. Significant recoverable reserves remained within the well’s drainage area. The
combined ChevronTexaco and
Schlumberger stimulation team examined several possible damage mechanisms to explain the loss of oil production. Drilling-induced damage— Filtrate inva- sion; invasion of a calcium carbonate bridging agent, polymer, starch and drilled solids; and fi l- tercake plugging of the screen and sandface could go unnoticed initially in a horizontal well. However, such damage can create localized pro- duction areas, which can eventually lead to early water breakthrough, loss of screens and acceler- ated fi nes production. Completion damage— The collapse of the shale section prevented a complete gravel pack, so the fi ltercake and mud removal in the section beyond the damage was probably extremely poor. Swelling clays— X-ray diffraction mineral-
ogy from a core sample showed that the volume of swelling clays, such as smectite, was too low to be a damage mechanism. Inorganic scale— Damage from barium sulfate [BaSO4] was expected to be small, but CaCO3 scale could be a major source of damage. Limited data were available to quantify the volumes of scale.
Water-holdup problems— NODAL production system analysis results showed that water cut in this fi eld must exceed 50% to create a signifi cant impediment to production. The measured value of 20% shows this is an unlikely damage mechanism. Fines migration— X-ray diffraction results indicated the presence of migratory clays such as chlorite and illite along with mobile quartzite particles. A pump-in test supported fines as a damage source. Permeability increased during the pump-in— that is, reverse-flow— period, as compared with the permeability during produc- tion. Further evidence of fines migration was found in the decreasing oil production with increasing water production, since water can destabilize fines and cause them to migrate.20 Finally,
the formation is unconsolidated, and other wells in the area had experienced fi nes migration.
This analysis indicated that the treatment
This treatment, which stabilized iron and also removed CaCO3 scale, was followed by
had to remove damage possibly caused by drilling, inorganic scale and migration of clays and quartzite particles. The proposed treatment started with jetting a chelating agent using a coiled tubing string with a high-pressure noz- zle.21
66
Oilfi eld Review
Oil production, B/D Wellhead pressure, psi 0
Gamma Ray API
200
MD, ft
Volume 0 vol/vol
Shale Sand Oil
Water 1
Water production, B/D
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