- Assume that you have been injecting at a constant rate into a fully penetrating well and that you then shut it in. The pressure is often reasonably assumed to be constant at a distance re from the wellbore, due to the interaction of the injector and surrounding producers.
- Notice the difference between all producers and an injector-producer combination. For producers, there is a boundary between the wells where no flow takes place. For an injector and a producer, there is often a boundary condition midway between the two where there is a contour of constant pressure. Since the pressure distribution is different between these two cases, you need to calculate the average reservoir pressure differently than you would for producers alone.
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- In conventional production situations, you can often approximate the average pressure, without much error by doing a Horner plot and extrapolating. This can result in significant errors when you analyse falloff data from an injector. This extrpolated value, a false pressure, p*, can be considerably different from the average reservoir pressure. It is necessary to calcuate the average pressure from relationships between the false pressure and the average reservoir pressure.
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Horner's method does not apply to closed or multi-well systems without correction. In closed, single-well reservoirs, the average pressure generally is above the extrapolated false pressure, p* (considering injectors). There are situations where the opposite can be true. It is generally most appropriate to use the Matthews-Brons-Hazebroek Method. Other methods are available and may have specific application. The shape of the injection area and the configuration of surrounding injectors and producers can make predictions difficult.
- Remember that many of the simple standard methods only apply to unit mobility situations.