Obtaining formation characteristics in a tight-gas environment is highly challenging: formation tester tools run during open-hole logging are often unsuccessful, and conventional well tests are not appropriate as no gas flow is commonly observed before hydraulic fracturing. This paper presents a new well testing methodology adapted from techniques used in the salt industry and successfully applied to Tight Gas Reservoirs, using only wellhead pressure gauges and surface flow meters without downhole tools. It delivers an injectivity index profile along the open hole, allowing better identification of the zones with the best potential and optimization of the hydraulic fracturing strategy. The test typically consists, after having run a tubing string, in continuously displacing the annulus mud by a heavier and less viscous fluid such as brine. During the test, the mud/brine interface rises up along the borehole as a result of pumping brine down the tubing at constant pressure and circulating out mud from the annulus. As viscosities and densities are contrasted, brine and mud injection rates in a given layer are also expected to be contrasted. The differences between brine and mud flow rates provide a continuous injectivity indicator along the wellbore. A different procedure, involving stationary step-by-step measurements, can also be performed. The fluid interface is displaced to a predetermined depth, at which step the entire wellbore is pressurized and a fall-off resulting from fluid leak-off across the open hole is recorded and analyzed to derive a layer productivity index.
To date, three well tests have been carried out in various types of wells, showing good correlations with conventional logs. To go a step further than a qualitative tool, a deconvolution of the time-dependent and interface-depth-dependent permeation flows is necessary. An inversion process accounting for the injection history in the various layers is being developed with the objective of deriving a continuous kh and skin profile. This new method still needs to be qualified against conventional kh data. In particular, it must be demonstrated that the test provides access to the kh of the undamaged zone of the wellbore.