FR2652381A1 - Hydraulic fracturing and thermal stimulation with solvent process for the dissociation of gas hydrates with a view to exploitation of the natural gas produced - Google Patents

Abstract

Hydraulic fracturing and thermal stimulation with solvent (F.T.S.S.) process for the exploitation of natural gas reserves of a gas hydrate deposit. The invention relates to a process for the dissociation of gas hydrates of a deposit and to the exploitation of the natural gas produced. It consists of the hydraulic fracturing of the gas hydrate deposit and of the circulation of a hot solvent (dilute methanol or brine) through the either horizontal or vertical induced fracture. The process can be improved by creating one or more wells at the end of the fracture in order to control the stimulation liquid flow which can be recycled, and a production well independent of the injection well. The process according to the invention can moreover be applied to the exploitation of a heavy oil deposit underlying the natural gas hydrate deposit.

Description

 The present invention relates to a process for the dissociation of the gas hydrates of a deposit and the exploitation of the natural gas produced.

 Natural gas hydrates are stable crystalline compounds under certain conditions of high pressures (10 to 2.10 MPa) and low temperatures (100 to 400 K) and located in the shallow crust.

 Large quantities of methane are trapped in small volumes of hydrates, so this represents a substantial and in-situ source of natural gas.

 In order to exploit the natural gas of this resource, it is necessary to dissociate the structure consisting of water molecules. So far two methods have been proposed: the process of depressurization and thermal stimulation.

 Figure 1 explains these two processes by locating them in the phase diagram of hydrates. The process of depressurization (Fig. 1 arrow A) consists in digging a simple well in the hydrate deposit, so that the dissociation is done by simple depressurization. This method has the big disadvantage of being too slow. Several years are necessary before obtaining a sufficient amount of natural gas.

The thermal stimulation process (Fig. 1 arrow
B) involves injecting hot water or steam into a simple well in cycles. The defect of the method lies in the geometric prohibition: indeed there is no sufficient space inside the very impermeable hydrate deposit in order to be able to receive a large quantity of liquid.

 This is why the process according to the invention makes it possible to remedy the two mentioned drawbacks and thus to effectively solve this problem of in-situ dissociation of gas hydrate deposit. It consists of the hydraulic fracturing of the gas hydrate deposit and the circulation of a hot solvent through the fracture.

 This fracture may be induced before the injection of the fluid and thus maintained by a proppant or directly using the injected fluid. The choice being guided by the pressure-temperature characteristics of the fluid. The second seems to be the easiest to implement.

 The injected fluid will be hot water or steam at 1500C provided if possible with a solvent: salt (from brine water of sufficient salinity: 25%) or methanol (at a concentration> 2% to have the same effect as brine).

 Methanol, although more expensive, is much more efficient than brine. Its use will be preferred in the case of continental exploitation unless there is a geothermal source nearby.

 The arrow C of FIG. 1 makes it possible to understand why the injection of an inhibitor is important in the dissociation of the gas hydrates, it makes it possible to retreat the phase diagram in an interesting way.

 In the case of a reservoir of natural gas hydrates always very impermeable, the fracture induced will be either horizontal or vertical. according to the depth H of the fracture vis-à-vis the critical depth Hc. The latter is the depth for which the vertical stress surpasses the horizontal stresses and depends on the internal pressure of the reservoir, the mechanical characteristics of the hydrates and the tectonic stresses in situ.

 If H <Hc, we have a near-horizontal fracture.

 If H> Hc, we have a vertical fracture which is the most common.

Figure 2 shows the process of dissociation by the STSF process. Arrived in the fracture the hot solvent will have two actions on the hydrated medium
1 - Thermal conduction and decreasing action in time of the inhibitor. This allows the direct and efficient dissociation of gas hydrates.

 The shape of the dissociation front is shown in Figure 3, in the case of a horizontal fracture and in Figure 4, in the case of a vertical fracture.

 2 - Effect of leak-off: sort of diffusion of the liquid in the dissociated medium which improves the action of the inhibitor.

The production of natural gas obtained after dissociation of the hydrates can be carried out in two different ways
* either directly from the injection well,
* by creating one or more wells at the end of the fracture.

The latter solution also has the advantage of having control of the flow of liquid through the fracture: indeed it would be possible at the same time to pump the cooled liquid from the production well and to recycle it in the well. injection after heating and supplement.

Figure 5 shows the performance of the process
STSF (C) with respect to the simple steam injection process (STEAM A) and simple injection of hot brine without fracture (Hot Brine B).

The E.E.R. (Energy Efficiency Report) and the cumulative volume of natural gas produced by the S.T.S.F process are on average three times higher than the other two processes.

 Note that the process according to the invention can be applied to the exploitation of a heavy oil deposit underlying the gas hydrate deposit. We can therefore bring to the process many variations and modifications without departing from either its frame or its spirit.

Claims (10)

 2 - CLAIMS  1) Thermal stimulation process with or without solvent after hydraulic fracturing (S.T.S.F) for the exploitation of natural gas reserves of a gas hydrate deposit.  2) Method according to claim 1, characterized in that the fracture is induced by stimulation fluid.  3) Method according to claim 1, characterized in that the fracture is completed by a proppant before the injection of the stimulation fluid.  4) Process according to any one of the preceding claims, characterized in that the solvent is diluted methanol at a stifling concentration, (> 2% indicative figure depending on the reservoir).  5) Process according to any one of claims 1, 2, 3, characterized in that the solvent is brine containing a sufficient amount of salt (> 5% indicative figure).  6) Process according to any one of the preceding claims characterized in that the diluted solvent is hot and a temperature (> 100 C).  7) Method according to any one of the preceding claims characterized in that the fracture is induced vertically.  8) Method according to any one of claims 1, 2, 3, 4, 5, 6, characterized in that the fracture is induced horizontally.  9) Method according to any one of the preceding claims, characterized in that one or more wells is established at the end of the fracture in order to have a control of the flow of the stimulation liquid through the fracture which can be recycled, and the obtaining a production well independent of the injection well.  10) Process according to any one of the preceding claims, characterized in that it is applied to the exploitation of a heavy oil field underlying the natural gas hydrate deposit.