Field Experiment and Data Analysis of a Constant-Head Injection Test with Skin Effects in a Low-Transmissivity Aquifer

Abstract

In a confined, silty sand aquifer of low transmissivity, constant-rate pumping can rapidly dewater the pumping well without creating measurable drawdown at nearby observation wells. To overcome this difficulty, a constant-head test is performed by overflowing the test well by water injection. The difference between the injection rate and the overflow rate directly gives the cumulative volume Vw(t) entering the aquifer, which is characteristic of the aquifer hydrogeologic properties and can be used for parameter estimation. The field experiments include two constant-head injection tests, CHIT1 and CHIT2, performed at two different wells, respectively. Semilog plot of t/Vw(t) of CHIT2 indicates little or no skin, and the two unknown parameters of the storage coefficient and transmissivity can be uniquely determined in a similar way by the well-known Jacob semilog method. Semilog plot of t/Vw(t) of CHITl, however, shows the ex istence of well skin, and a curve matching method is developed for estimatiog the four pertinent parameters. Among them, the aquifer transmissivity can be uniquely determined while the skin transmissivity, the skin thickness, and the storage coefficient are obtained in appropriate ranges. To avoid non-unique estimates of aquifer parameters, therefore, the test well should be carefully constructed with complete and thorough well development to prevent skin region from happening. Due to the limitation of the site hydrogeology, only slug tests can be performed for the sake of validating transmissivity. The transmissivity values determined from the CHITs and from the slug tests are in good agreement, supporting the usefulness of the CHIT. The aquifer transmissivities estimated from CHITl and CHIT2 are within 14% difference, indicating that the aquifer is relatively homogeneous in the neighborhood of the two test wells.