In the Ordos formation of Inner Mongolia, the Sulige gas field is one of the largest discoveries ever made in China. The development of the South Sulige block (equivalent to roughly 10% of the Sulige field as a whole) was launched in 2012 under a PSC with Total. The target plateau production of 8.2 Mm3/d (or 3 Gm3/yr) is expected by 2022. Yet South Sulige has presented a number of technical challenges: it is a tight gas field consisting in deeply buried, low-permeability, braided-channel reservoirs. As partners to field operator PetroChina, we are deploying an array of high-performance drilling and hydraulic fracturing technologies to maximize well productivity.
Geological challenges to productivity
The tight sandstone reservoirs of Sulige South are part of an extensive fluvial-lacustrine sedimentary system dating from 250 million years ago, with a burial depth of 3,500 meters and layer thickness ranging from 5 to 20 m according to the unit. These multiple reservoirs exhibit significant heterogeneity and mediocre petrophysical properties, including permeability of less than 1.0 mD and low porosity of 7% on average.
The complex geology and burial depth of this group of reservoirs make it difficult for seismic to identify the geometry and areal extent of the reservoirs. The sedimentary environment consists in a braided system with varying degrees of overlap and more-or-less interconnected reservoirs. Reservoir models are continually refined by reworking the analyses of sediment geology to screen for sweet spots. The challenge is to site as many wells as possible in the connection zones to produce sufficient amounts of gas.
Our Solutions for Improving Recovery
Faced with the limits of reservoir models, optimizing the productivity of this field relies largely on the effectiveness of operating techniques. Accordingly, backed by PetroChina, we are deploying solutions decisive for optimizing the production of the reserves. In fact, our techniques have been so successful that our partner has even decided to deploy them on other blocks of the Sulige field.
- Slim-hole drilling uses small-diameter boreholes. It is being applied systematically to all wells of Sulige South pursuant to successful field test results. This technology allows for a higher rate of drill bit penetration, decreases the volume of cuttings by half and reduces the quantities of drilling fluids and cement required. In other words, slim-hole wells limit rig time and curb the costs and environmental impacts of drilling. Following a field test on eight wells in 2015, slim-hole drilling was extended to two-thirds of the new wells planned for 2018 and will be deployed systematically in 2019.
- Factory drilling sharply reduces both the time and the costs of drilling. On the strength of our renowned experience with the “industrialization” of drilling processes, acquired in offshore as well as unconventional domains, we are implementing a high degree of standardization of our drilling and completion techniques on South Sulige, thanks to the efficient contribution of our operating partner and its contractors in the field. The wells are arranged in clusters of 9 wells on 256 pads. We have 10 to 15 mobile but non-winterized drilling rigs operating simultaneously at a rate of two per pad between March and November. By continually optimizing each individual step of well construction, we will be able to further shorten the time required to bring each well on stream.
- Specific hydraulic fracturing design. Our team seconded to China defined a fracking design, selecting proppants and fracking fluids tailored specifically to the geological properties of the field to improve the fracture network and conductivity. This expertise further contributes to boosting well productivity.
- Widespread use of the velocity string. Well productivity declines quickly once the wells are brought in due to pressure drop and liquid-loading issues. To remedy this problem and stabilize production, our solution has been to run a velocity string into the production tubing. Velocity strings were installed in 25 wells in 2015. The results are conclusive: installing this small-diameter tubing inside the well completion indeed reduces the flow area and restores a sufficient gas flow velocity to boost productivity. Accordingly, the initiative was applied to some 130 wells in 2017, to bring production closer to target. Going forward, all new wells will be equipped with a velocity string. It is worth noting that the installation of velocity strings required the design and construction of a bespoke structure to expediate operations.
At the same time, we are considering other possible avenues of improvement and innovation, such as simultaneous operations or horizontal wells. All these options need to be studied (or re-examined) with the aim of maximizing production at minimal cost and in full compliance with the stringent safety and environmental protection conditions that local regulations impose.
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