Formation damage, sand production and a HP/HT sour gas environment are major challenges in the field development of the Sawan gas reservoir in Pakistan. Meeting individual well productivity targets (average of 80 MMscfd) requires minimal-restriction (large bore or monobore) completions, minimizing formation damage via under-balance perforation and maintaining sand production below tolerable limits.
The reservoir sands are extremely heterogeneous with many thin, weak layers. Sand control completion concepts are based either on internal gravel packs (IGP) or, if the weaker layers are less extensive, on selective perforation. To avoid having to kill the wells (which can lead to significant formation damage) the completions are run prior to perforating. This meant that the sand control decision for each development well had to be made in 3 to 4 days - between logging the well at TD and running the completion.
The key elements of this "real time" sand management strategy were:
the application of Fuzzy Logic computing techniques to correlate wireline log responses with core measurements, which enabled a field-calibrated, continuous sand failure prediction tool throughout the reservoir intervals;
coupled well performance and geomechanical models to evaluate if selective perforation could safeguard well deliverability yet ensure sand-free production, standardization of critical completion equipment and simplified nipple-less completion design which provided the necessary operational flexibility, lowered overall completion cost, and reduced the risk of equipment failure.
under-balanced through-tubing perforation with coiled tubing to minimize formation damage.
This integrated completion and sand management strategy has delivered well production rates in excess of 100 MMscfd with sand production significantly below tolerable limits. It has also has created substantial cost savings and reduced completion failure risks compared to conventional cased-hole sand exclusion completions.
The Sawan field is located in the South West Miano block in the Thar desert in the southeast of Pakistan (Fig 1).
The Lower Goru "C-sand" formation in Sawan is typically around 70 m thick. The sands are classed as sublith-arenites to lithic arenites, with a high content of partially altered basic volcanic rock fragments and pore-lining or pore filling iron chlorite cement. The chlorite, which comprises some 80% of the total clay fraction, is known to cause severe formation damage problems when contacted with incompatible drilling, completion and kill fluids. The chlorite prevented a subsequent, significant precipitation of quartz cement, which would tend to reduce the porosity and increase the strength of the rock.
The reservoir sands are overlain by transgressive, siderite cemented, shaly silt to very fine-grained chamosite sandstones.
Initial reservoir pressure is 5386 psi at the GWC at 3295 m TVDSS. Reservoir temperature is 352°F. Gas gravity is estimated as 0.642 SG and water density at 56.44 lb/ft3.
Exploration well Sawan-1 (S-1) was drilled between October and December 1997. The well encountered 70 m of gas bearing, good quality reservoir. Core and logs identified some thin, weak formation intervals and stringers. No GWC could be identified from the open hole logs, although DST results from zones below the main reservoir indicated the presence of moveable water.