On China’s Sulige field, an unprecedented solution is being implemented since 2016 to optimize the productivity of the field’s unconventional tight gas reservoirs: the LedaFlow® multiphase flow simulator and the Reveal® reservoir modeling tool have been coupled numerically to model the simultaneous transient phenomena and interactions between a reservoir and a well. This is a world first using a commercial reservoir modeling software and truly advances our expertise in production optimization for unconventional fields.
Greater accuracy when accounting for transient events occurring in both the reservoir and the well
Reservoir models are always based on a well model derived from pressure loss tables that assume steady-state behavior in that well. In this approach, all production reaching the bottom of the well is considered to be lifted to the wellhead without downhole accumulation. Likewise, flow simulators such as LedaFlow® take transient events into account, but only those occurring inside the well. The inflow rate is usually based on a constant productivity index (PI). This is clearly an approximation, but until now, there has been no satisfactory means of integrating the transient flow events of the reservoir with those of the well.
The Sulige field is an example of cases where classic well modelling based on pressure loss tables is inadequate: the low reservoir permeability results in low gas rates into the well – below the so-called “critical” rate – meaning that liquids cannot be lifted all the way to the wellhead, and leading to the gradual onset of liquid-loading. Gas production becomes unstable, with production spiking and then gradually declining over a period of 30-40 days, typically until the gas rate becomes extremely low. This is followed by a new production spike. Indeed, when the gas flow rate drops far enough, liquids accumulating downhole drain naturally back into the reservoir, allowing a new slug of gas to flow into the wellbore.
Ultimately this instability causes transient events in the reservoir. The interaction of transient phenomena in the wellbore and the reservoir cannot be represented by a classic reservoir model.
Representing simultaneous transient phenomena
The coupling is performed by a workflow managed by Resolve®, the software integration platform developed by Petex. Through iterative exchanges of data (pressure, PI, gas rate, liquid rate), consistent and concurrent simulations are obtained by synchronizing LedaFlow® (with time steps typically on the order of one second) with Reveal (with time steps ranging from 0.5 to 6 hours). This approach reproduces the transient phenomena occurring in both the reservoir and the wellbore as well as the interactions between them.
The coupling is generally performed for a duration of two months. Implemented at regular pluri-annual intervals, the coupled model gives a more meaningful representation of transient reservoir-wellbore flows ; it enables to fine-tune the production predictions derived from a classic reservoir model. It is worth noting that specific workflows have also been developed to predict and optimize well productivity in a cyclic scenario (i.e., alternating periods of production and well shut-in).
Recommendations for operators to optimize average well production
Cyclic production is a technique conventionally employed on low-productivity wells. Nevertheless, truly optimizing cyclic production would require tedious time-consuming well tests. The LedaFlow®-Reveal® coupled model of cyclic production over a two-month period gives an idea of a representative average production from a well. Various strategies for well shut-in (according to a minimum gas flow criterion) then opening (according to criteria for shut-in duration or change in shut-in pressures) can be simulated and compared. Recommendations for managing wells in cyclic production can then be conveyed to operators. Ultimately, the actual optimization of cyclic production on Sulige could require to install systems to automatically open and shut-in wells.
With the LedaFLow®/Reveal® coupled model, Total – along with its partner PetroChina – confirms its role in spearheading the optimization of well productivity and production strategies. The coupled model is an effective way to represent the interaction between reservoir transients and wellbore transients. It has many applications outside the unconventional domain, particularly to model wells restart, coning phenomena or instabilities in gas-lifted well.
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