Laboratory Study of Gas Trapping in Foam-Acid Diversion

Abstract

A series of Berea coreflood experiments modeled the process of foam-acid diversion for well stimulation. Foam was injected to steady state, followed by injection of liquid to represent acid. Pressure gradient was monitored along the core and average liquid saturation was determined by weighing the core continuously during the experiments. This approach provides direct data on liquid saturation and liquid relative permeability during foam injection and liquid injection after foam. Water saturation increased by a larger amount during post-foam liquid injection than estimated in previous studies. The liquid relative-permeability function krw(Sw) obtained during foam injection was reasonably accurate during post-foam liquid injection, until gas dissolution into the injected liquid became significant. The ultimate trapped-gas saturation during liquid injection appears to depend on previously injected foam quality; this may require a revision of models for the acid-diversion process. It appears that a sufficiently high foam quality (injected gas fraction) can eliminate the detrimental decline in pressure gradient observed during liquid injection in earlier studies. This may alter previous ideas about the optimal foam quality for foam-acid diversion. Introduction Matrix Acidizing Process. Foams have been used routinely in matrix acidizing treatments in the petroleum industry, but with inconsistent results.1–7 Matrix acidizing is a treatment for damaged sandstone or carbonate formations. Formation damage can be caused by drilling, completion, workover, or production processes. A damaged formation has plugged or constricted pore spaces, reduced permeability and reduced productivity. In an acidizing process, acid is used to dissolve material in the matrix, thus increase porosity, permeability and productivity. A formation may have layers with different porosity, permeability and rock types. Layers may also differ in degree of damage. In matrix acidizing, all damaged layers need to be treated, especially the most damaged layers. However, fluid naturally flows into the most-permeable, least-damaged layers, and may leave less-permeable and more-damaged layers under-treated. To solve this problem, foam is used to partially block the high-permeability and undamaged layers and divert acid into the less-permeable layers. Foam for Acid Diversion There are several designs for foam acidizing processes. In particular, one may pre-flush the formation with surfactant, inject foam, and then inject foam-compatible acid. A foam-compatible acid must also contain surfactant to stabilize the foam.8 Multiple slugs of foam and acid may be used. Or, instead, one may inject a surfactant pre-flush followed by continuous injection of foamed acid. This study focuses on the first process, i.e. injecting foam followed by acid. Many studies9–14 agree that foam does not alter water (or acid) viscosity or the relation between water relative permeability krw and water saturation Sw in steady foam flow. Foam does directly reduce gas mobility, thus indirectly reducing water saturation, relative permeability and mobility. Foam reduces gas mobility in part by trapping a large percentage of gas in place; up to 80–99% of gas is trapped even if foam flows at high pressure gradient.14,15 Foam reduces gas mobility also by increasing the effective viscosity of flowing gas.16–19 These two effects are related: both depend on capillary forces that give foam an apparent yield stress and trap bubbles in place.19–23 The presence of foam in a high-permeability or undamaged layers reduces liquid saturation there, reducing relative permeability of liquid krw(Sw), and thus reducing acid flow into the foam-saturated layers. On the other hand, foam is less stable in low-permeability layers.24,25 Therefore it is possible to block high-permeability zones preferentially during foam injection, and maintain foam there during acid injection. At the same time, only weaker foam is present in low-permeability or damaged zones. Acid can then be diverted into the lower-permeability or damaged zones without zonal isolation. Use of a surfactant pre-flush also increases diversion by helping to place more foam in high-permeability or undamaged layers.8

Evaluation of the effects of CO₂ injection in gas and foam phase on different rock and fluid properties during an Enhanced Oil Recovery (EOR) operation.

“…En la inyección de espuma de CO2, el control de la movilidad mediante la reducción de viscosidad mejora la eficiencia del barrido, dando como resultado un mayor factor de recobro y una producción más efectiva. Además, la espuma puede suavizar heterogeneidades del yacimiento [40].…”

Miscible Displacement Oil Recovery

“…The first mechanism is related to the movment and re-arrangement of bubbles due to the local gradient in the surfactant concentration and, therefore, the interfacial tension. The surfactant movement within the liquid film (Lamellae) lowers the surface tension between the two phases (liquid and gas) that slows down the bubble motion and causes an increase in the gas phase effective viscosity [120][121][122]. The second mechanism that reduces the gas-foam mobility is gas trapping [123,124].…”

Foam front propagation in anisotropic oil reservoirs

“…As alluded to above, foam improved oil recovery generally proceeds by flooding an oil reservoir with surfactant solution and then injecting gas under a specified driving pressure (the so-called "surfactant alternating gas" process [7][8][9][10]21]). A foam front forms at the boundary between the surfactant solution and the gas, and this advances into the reservoir.…”

“…Producing foam via co-injection of gas and surfactant solution therefore requires high pressures. This situation can be mitigated by injecting first surfactant solution and then gas (the so-called surfactant-alternating-gas process [7][8][9][10]), and this is the situation that we study here. Foam (i.e.…”