A completed well is a sophisticated industrial-scale plumbing system, designed to transport fluids between subsurface reservoirs and the surface safely, productively and profitably. Injectors transport fluids one way, and producers transport more valuable fluids the other.
Well system fusion
Before the insertion of this grand plumbing scheme into the earth, the targeted subsurface reservoir and the fluids contained within it existed for millennia in a state of relative equilibrium. However, when the human-made tubulars, cement sheath and other well completion elements were “fused” with the subsurface by the well construction processes, this stasis was dramatically transformed into a complex and turbulent dynamic state.
This fusion between the human-made materials of the well completion and the natural materials of the earth, together with the dynamic interplay that now exists between the two, is what TGT Oilfield Services calls the “total well system.”
The well system includes that previously elusive volume of earth that exists beyond the wellbore, in the outer periphery of the well completion and the cement sheath that surrounds it—the so-called “well-to-reservoir interface.”
Understanding the behavior of fluids here, and specifically the “flow” of fluids, is crucial to understanding the productive behavior of the entire well system. This is one reason why TGT Oilfield Services developed through-barrier diagnostics, which reveal flow behavior throughout the well system, from the wellbore through the completion and to its outer extremities where it connects intimately with the reservoir.
An imperfect world
In a perfect world, the well completion behaves according to its design and transports the right fluids to and from the right place in the subsurface. Moreover, in the same perfect world, the reservoir surrenders or receives the right fluids, and the total well system delivers safely, productively and profitably according to plan.
However, imperfections corrupt this ideal relationship and forces conspire to undermine the system. Imperfect cement seals, degraded packers, worn out valves, corroded pipe, near wellbore fractures and other barrier failures collude to open unwanted flow paths throughout the well system. As a result, essential fluids are diverted, sustained annulus pressures can dangerously manifest and, ultimately, producers or injectors will not behave as expected or underperform.
A classic example of this occurs in injector wells. Petroleum and reservoir engineers determine that if water is injected at a particular pressure, then subsurface target zones will receive a certain volume of water over time. If the predicted flow rate is not observed, then either something is wrong with the assumptions and calculations or something is wrong with the well system—or both. Even worse, the predicted flow rate might be within range, but the water might not be reaching the target. The latter scenario is particularly insidious because it may be weeks, months or longer before an alarm is raised.
TGT has diagnosed thousands of injector wells and, in the majority of cases, has revealed unwanted flow paths behind the production casing, under- and overperforming target zones, and “thief zones” that effectively “steal” water from its intended destination.
Consider the injection well case shown in Figure 1. Conventional borehole flow diagnostics using production logging techniques (PLTs) tell the operator that most of the injected water is reaching the top half of the target reservoir unit (A3), and the rest is entering the lower half (far right track labeled “borehole flow profile”).
However, through-barrier spectral diagnostics by TGT reveal the true picture of what is happening with the well system. In reality, only 25% of the injected water is entering the target reservoir unit. The rest is channeling upward to a shallower unit (A2) from 210.3 m (690 ft) to 158.5 m (520 ft), probably though an imperfect cement sheath behind casing. A smaller amount is channeling downward.
This is a serious issue from both a well and reservoir management perspective. Not only is the target reservoir not receiving enough water to fulfill the field injection strategy, but 75% of the injected water is being wasted and potentially causing water breakthrough issues at other wells, compounding the loss. This essential information directly impacts well performance and potentially fieldwide management decisions.
Harnessing acoustic, thermal energy
TGT’s spectral diagnostics harness acoustic and thermal energy to locate and quantify fluid flow behind well barriers, thereby providing a complete picture of flow dynamics and pathways throughout the well system. High-fidelity sound recordings and processing technology deployed downhole locate flow activity by capturing and analyzing the characteristics of sound energy generated by pressurized fluid passing through well system restrictions, such as cement channels and reservoir entry points.
The position and relative intensity of the resulting spectral signature indicate the precise locations of flow activity (see the middle track of Figure 1 labeled “spectral injection”). This information is then used together with other well system data to guide a powerful and unique flow modeling engine that transforms precise thermal profiles into flow rates. The result is a behind-casing reservoir flow profile, which can be used in combination with the borehole flow profile to enable better well management and field management decisions (see right-hand track labeled “reservoir flow profile”).
Well barrier imperfections exist in all well types, so similar “unwanted flow path” scenarios exist in production wells too.
The case shown in Figure 2 is a deviated production well exhibiting a very high water cut of greater than 90%. Identifying the source of high water cut is one of the most urgent priorities for petroleum and reservoir engineers to resolve.
Whereas the PLT-derived borehole flow profile can only measure flow entering the wellbore in front of the perforated interval (A2), the spectral signature map indicates significant flow activity behind casing at several other producing intervals, namely A3, A4 and A5, and to a lesser extent at A1. Given that these intervals are known to be water-filled, the operator can confidently conclude that more than 60% of the produced water is coming from these zones. Knowing the exact locations of the source, the operator can take appropriate action to seal off the unwanted flow paths.
Good bond, bad seal
In the case study, the operator concluded that water from these zones was channeling through an imperfect cement sheath. Even though the azimuthal cement map and cement bond log indicated that the cement sheath had good mechanical coverage and a good bond with the casing, the cement was not providing a hydraulic seal. This specific aspect underlines the importance of verifying both barrier condition and barrier sealing performance when deciphering flow dynamics around the well system and eliminating unwanted flow.
Completing the picture
Conventional technology, such as PLTs, helps operators understand flow dynamics within the wellbore. However, this information does not always align with what is happening beyond the wellbore—beyond casing and cement at the reservoir interface. Evaluating the well system with through-barrier diagnostics is the only way to understand what is happening in the well system. Armed with a complete picture, the operator can confidently make better decisions to ensure the well system delivers the right fluids to the right place, safely and profitably for the entire productive life of the well.
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