The search for practical engineering tools to better manage water injectors has been ongoing for many years. Simple analytical solutions have proven inadequate, and most reservoir simulators do not explicitly model water injectors. This paper proposes a methodology of using detailed and coupled fracture and reservoir modeling to ensure proper injection start-up procedures, and managing injection rates to avoid out-of-zone injection. This is combined with simple rate-pressure data to understand injection behavior, and from that control the process.
Water injection, in particular produced water injection, in most cases creates a fracture. As is well established , temperature effects are important, because the cold injection fluid reduces the temperature of the near well formations, leading to a reduced minimum in situ stress. If the fracture remains within the cooled region, the reduced stress may be a significant factor in reducing fracture height growth, or in other words, for the confinement of the fracture to a given formation. As a result, simulations of water injection need to take fracturing and the temperature effects on fracturing into account.
In this paper we briefly review some elementary aspects of the response of a fractured injector. We proceed to show field examples of fracture growth, with particular emphasize on its episodic nature.
Next, we describe a new model for the simulation of water injection, which includes thermal effects and plugging due to dirty injection water.
The main part of the paper is a discussion of some simulations, focusing in particular on how the rate schedule of injection start-up may be used to influence the height growth of the fracture. This is of central relevance for many field cases, where it may be imperative to constrain fracture height growth, e.g. to avoid growing into a thief zone.
In terms of reserve recovery, this is crucial. If the fracture grows into high permeability layers, those layers begin to accept more injection. This causes preferential cooling in those zones, reducing closure stress there. The fracture then migrates there, further increasing injection into that zone, causing more cooling, etc., etc. Ultimately, this causes early water breakthrough and lost reserves!
With modern data acquisition systems, extensive datasets are normally available for all or many of the injectors in a field. Hence it is important to find an effective way of screening the data, in order to single out the features for further study. A plot of large amounts of data in a pressure versus rate plot may be an effective method, in particular if color coding is used to separate different time intervals.
Before showing real data, we briefly recap the main features of injector response in the pressure versus rate domain. Fig. 1 schematically shows the response of an injector working both in the matrix and fracture regimes. The full line describes an initial situation, in which the injector injects into matrix up to a given pressure where the fracture opens, and an increased injectivity results. The dashed line shows the change resulting from a stress reduction due to reservoir cooling. To a first approximation the slopes of the matrix and fracture injection lines are unchanged, but the intercept takes place at a lower pressure due to the reduced reservoir stress.
The dash-dot lines show the effect of pressure build-up around the injector. This affects both the matrix injection and the fracture injection. Note how the intercept occurs at a lower injection rate. This is because the matrix injection responds directly to the pressure, while the initiation of fracture injection responds to the changes in the leakoff stress, which is general smaller.
Fig. 2 shows the effects of changing the injector's choke. The curved full lines correspond to various choke settings, while the straight lines represent two different values of injectivity. If, for a fixed choke setting, the injectivity changes for some reason, the response will stay on the fixed choke line. Thus, the path in the p-q plot is fully determined by the choke setting, and in itself cannot say anything about the mechanism leading to the change in injectivity.