Developed and patented by Total, Well Test Log (WTLog) makes it possible to use surface measurements to obtain an injectivity index profile along an open hole. The technique involves pumping two immiscible fluids of different viscosity into the annulus one after the other while maintaining constant wellhead pressure. A numerical model has been developed to simulate injectivity measurements and obtain a continuous permeability log. This simple, innovative method will make it possible to monetize unconventional fields at a lower cost, offering an effective response to the economic viability issues involved in developing this type of resource.
How It Works
The less viscous fluid can move through the media in the lower zone, while the more viscous fluid’s ability to move through the media in the upper zone remains insignificant. The injection rate increases when the interface between the two fluids reaches a more permeable layer and decreases when the interface comes to a less permeable layer. The injection and withdrawal rates are measured at the wellhead and the differences between these two rates are used to obtain an injectivity index along the open hole in real time.
Assuming similar behavior during production and injection, the formation’s productivity profile can be estimated from this index. In 2018, a laboratory test on a small-scale model demonstrated that it was possible to estimate a permeability profile from the inversion of the injection profile by interpreting the transient decay of the injection rate.
As a result, productivity and permeability profiles along the open hole can be determined using only wellhead pressure gauges and surface flow meters without downhole logging tools, a solution that drastically reduces data acquisition costs.
A World First in Argentina
Easy to implement in the field, WTLog can be used on all types of wells or reservoirs, for open-hole or cased-hole logging, no matter what the formation’s permeability or pressure. This is true, for example, for unconventional resources. We were the first to conduct an industrial-scale pilot run of WTLog in Argentina’s Vaca Muerta shale gas formation.
A first WTLog test was carried out on the APGe-411h horizontal well before hydraulic fracturing to identify formation heterogeneities. A second was performed after fracturing to assess the contribution of fractures. Obtaining these measurements cost ten to forty times less than with a production logging tool (PLT) or Fullbore Formation Microimager/Oil-Base Microimager (FMI/OBM) logging.
A Major Breakthrough for Unconventional Resources
WTLog is a major breakthrough that could be widely used on unconventional formations to assess hydraulic fracturing more effectively, thereby optimizing its efficiency.
- Before casing and hydraulic fracturing, WTLog methodology can be used to measure injectivity and estimate the geological formation’s permeability, locate heterogeneities and identify the zones with the best potential. It makes for a more efficient process than the current practice of uniform hydraulic fracture spacing.
- After hydraulic fracturing, WTLog can be used as a low-cost production logging tool to estimate each new fracture’s contribution to production.
- Data acquisition for unconventional wells is still limited to pilot well measurements, given the cost of a PLT or FMI/OBM. By optimizing each hydraulic fracturing operation at a very low cost, WTLog could soon be used on every well in a project. With hundreds of wells drilled, improving each one’s productivity would significantly increase a project’s return.
- WTLog could be quickly and optimally integrated and standardized in routine unconventional well drilling and completion during the cementing and cleaning phases with next to no impact on the project timetable.
From Unconventional to Conventional
Initially developed for tight and ultra-tight formations, WTLog can also be used in conventional environments alongside the usual well tests. It can provide data on permeability along the borehole at a lower cost and with less risk than a PLT, making it possible to identify fractured zones, drain holes and other heterogeneities. This is key information for understanding a reservoir’s dynamic behavior and optimizing a field’s development. In this way, WTLog would be a useful addition to conventional well tests that only provide an average of a formation’s permeability.
In 2018, a laboratory test demonstrated that it was possible to estimate a permeability profile from the inversion of the injection profile. We are currently looking for a pilot well on which we can conduct both a conventional and a WTLog test to validate the approach in real-world conditions.
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