A method is proposed for heating a hydrocarbons reservoir with a view to their extraction. The reservoir comprises in particular heavy oils, i.e. oils which are not very, or not at all, mobile. A distinction is drawn between several thermal assisted recovery methods. For example the “huff and puff” process, also called CSS (“Cyclic Steam Soaking”) described in the application U.S. Pat. No. 4,116,275 is a recovery method assisted by cyclic steam injection. This process uses a single wellbore and consists of three stages, repeated several times in succession: the steam is firstly injected via a wellbore (typically over a few weeks). This is followed by a period of a few days of soaking during which the steam condenses and passes its heat to the reservoir. Then comes the period of production via the same wellbore (from a few weeks to a few months). The wellbores are equipped with a bottom pump to remove the product.
Gravity drainage recovery methods have also been proposed, for example SAGD (Steam Assisted Gravity Drainage). The SAGD process involves drilling 2 parallel horizontal wellbores, one approximately five meters below the other. Steam is injected continuously through the upper wellbore. The injected steam heats the formation. If the permeability is sufficient, the liquefied bitumen and the water resulting from the condensation of the steam flow by gravity as far as the lower wellbore. The drained zone forms a “steam chamber” which expands as the bitumen is extracted. The oil produced is then replaced, in the formation, by the injected steam.
The methods of recovery by injection of a hot fluid comprise a first stage which involves circulating steam in the wellbores, so as to heat the reservoir around the wellbores. During this circulation phase, the injected steam condenses on the inner walls of the wellbore, which are initially cold. While condensing, the steam releases its latent heat and thus heats the inner walls of the wellbore. The heat is then transmitted by conduction to the part of the formation situated in the immediate proximity of the wellbore.
A satisfactory operation of the circulation phase is necessary to heat the reservoir homogeneously, and thus subsequently to allow an optimum production of hydrocarbons.
It is for example necessary, for safety reasons, to keep a bottom pressure below a limit value, which can for example be the fracturing pressure. The fracturing pressure is specific to each deposit and can vary typically from 10 to 150 bar. It is also desirable to limit the production of hydrocarbons during the circulation phase. In fact, the hydrocarbons would then be produced mixed with the condensates, i.e. in the form of an emulsion, which is harmful to the installation. In the case where the installation comprises several horizontal wellbores (case of a SAGD type configuration), it is also necessary to avoid the creation of preferential paths between the wellbores. Finally, the circulation phase must be relatively short, in order to allow the operators to rapidly access the production phase.
Moreover, as the circulation phase proceeds, constraints develop, in particular the bottom-pressure constraint. The bottom pressure depends on the reservoir, the weight of the column of liquid in the wellbore, and the steam injection pressure. At the start of the circulation phase, the injected steam has a tendency to condense on the walls of the strings: the extraction string is thus partially filled with a liquid condensate. As the bottom pressure depends on the weight of the column of liquid in the wellbore, there is a high bottom-pressure constraint at the start of the circulation phase: a small increase in the steam injection pressure makes it possible to reach the fracturing pressure. When the steam circulates in the injection and extraction string in gaseous form only, the bottom pressure falls for the same injected steam pressure. It is then possible to increase the steam injection pressure without the risk of reaching the fracturing pressure.
Control of the circulation phase is disclosed in several applications.
The process disclosed in application U.S. Pat. No. 5,931,230 is of the “huff and puff” type, with a wellbore equipped with a long string and a short string. Steam is circulated between the long string and the short string. Once the wellbore is in place, the hydrocarbons are heated by the continuous circulation of the steam as far as the horizontal part of the wellbore at a pressure below the fracturing pressure. While the steam is circulated, the steam injection pressure and the steam circulation flow rate (and thus, the bottom pressure in the wellbore) can be controlled by adjustable chokes situated at the surface on the injection string. However, this document does not say how the control is carried out.
The document U.S. Pat. No. 7,147,057 describes a recovery method comprising 2 partly horizontal wellbores, arranged one above the other and in which the steam circulates mainly in a closed circuit in the upper wellbore. This circuit is constituted by a looped conduit, with an inlet and an outlet at the wellhead. The steam circulation in the wellbore takes place via this conduit. Steam is injected into the upper wellbore at the conduit inlet. On contact with the cold walls of the conduit, the steam condenses and transmits its latent heat to the conduit, which is thus heated. Another conduit is provided at the lower wellbore, in which the hydrocarbon condensates are recovered. Valves are provided on the circulation system, so as to pass the steam into the formation under certain conditions in order to produce a steam chamber. During a first stage, which corresponds to the circulation phase, the valves are closed and the system operates in closed circuit. The circulation of the steam is continued until the temperature of the wellbore is at least equal to a determined temperature, for example the boiling point of water, which corresponds to a circulation in the form of steam throughout the circuit, without condensation of the steam on the walls.
As from this moment, the valves provided on the circulation device are opened, and the steam can then pass into the formation. The opening of the valves can be triggered by the temperature of the steam.
This device therefore makes it possible to control the quantity of steam entering the formation. However, it requires a specific string, i.e. with valves making it possible to control the injection of steam into the formation. The device described in the application U.S. Pat. No. 7,147,057 is a complex device, which will involve maintenance operations at the bottom of the wellbore, and a production shutdown in order to carry out these maintenance operations.
For the methods of recovery by injection of a hot fluid, there is therefore a need for a more precise control of the injection of the steam into the wellbore throughout the circulation phase, optimizing the steam injection flow rate throughout the circulation phase while still ensuring the safe operation of the steam circulation phase.