Application of DP_LAQ to Confined Fractured Karstic Aquifer Pumping Test Data
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This is a technical page on fractured aquifer testing. Click this link to see a nontechnical page on fractured aquifer testing.
Fractured Aquifer Pumping Test Selected
The purpose of this web page is to describe the results of an application of DP_LAQ pumping test analysis software (Moench, 136) to actual data. The pumping test selected for this study is described by Crouch (135). I selected this test because the quality of the data presented in the paper is good, and the pumping test provides an extreme case for exploring the applicability of the fractured aquifer capability of DP_LAQ. The case is extreme because the aquifer is karstic with properties that deviate considerably from the assumptions of the analytical solution used. An interesting aspect of the test is that an observation well (ZS-1) located 247 feet northeast of the pumped well had less drawdown than a more distant observation well (ZS-13) located 409 feet southwest of the pumped well. ZS-13 was reported to have penetrated “caves,” but ZS-1 did not.
Description of the Fractured Aquifer Tested
The aquifer is a buried karstic formation (San Andres Limestone, Permian) underlain by and interconnected with fractured sandstone (Glorietta Sandstone, Permian). The two hydraulically connected units are called the San Andres-Glorietta Aquifer. Most of the wells involved in the test encountered “caves,” which I suspect are driller's descriptions of open fissures or conduits formed by dissolution of limestone by groundwater under karst conditions. Karst developed when the limestone was exposed at the ground surface in late Permian or early Triassic time. These karst features are now buried beneath a thick aquitard composed of mudstone and siltstone in the lower part of the Chinle Formation (late Triassic). At the pumped well the San Andres-Glorietta Aquifer is 406 feet thick. It is underlain by more than 900 feet of interbedded sandstone, siltstone, limestone, and evaporites of the Abo and Yeso Formations (early and middle Permian) which act as a subjacent aquitard. The top of the aquifer is at a depth of 602 feet at the pumped well, but rises westward to crop out on an anticline about three miles from the pumped well.
Pumped Well Description
The pumped well has 18-inch casing to the top of the San Andres Limestone. Below the casing, 16-inch open hole extends through most of the 135 foot thickness of the San Andres Limestone, and 6-inch open hole extends 45 feet into the Glorietta Sandstone. The non-pumping water level was 120 feet above the top of the San Andres Limestone.
Observation Well Descriptions
Data are reported for four observation wells and an enclosed spring. The spring is on the aforementioned anticline 4.83 miles northwest of the pumped well. The observation well distances are 247 (ZS-1), 409 (ZS-13), 6,470 (ZS-101), and 15,950 (ZS-100) feet. They all encountered “caves” except ZS-1, and they all extend into the top of the Glorietta Sandstone. The present analysis only uses the two closest observation wells (ZS-1 and ZS-13).
Description of the Pumping Test and the Original Analysis
The pumping test and the original analysis are described by Crouch (135). The well was pumped at approximately 2540 gallons per minute for 13,380 minutes (9.3 days). Time-drawdown data were not reported for the pumped well, but the average drawdown was reported to be about 4.3 feet. Most of the drawdown occurred within a few minutes after pumping started.
The original analysis used the Jacob semilogarithmic plot method to analyze the pumping test data. This method yielded a transmissivity of 640,000 ft2/day and a storativity of 4.8E-4 from the ZS-13 data, and transmissivity of 750,000 ft2/day and storativity of 7E-2 from the ZS-1 data. The data from the more distant wells were not analyzed.
DP_LAQ Fractured Aquifer Pumping Test Analysis
The present analysis of the data used software titled DP_LAQ, which is described by Moench (136). DP_LAQ calculates type curves for pumping tests in fractured aquifer dual-porosity groundwater systems. It allows the user to choose between three cases for the simulation of dual-porosity. In the input file, the cases are labeled 4, 5, and 6:
4. Horizontal fractures separated by slabs of matrix rock,
5. Vertical and horizontal fractures with matrix rock in the interstitial cubes (simulated as spheres), and
6. Simplified matrix to fracture flow.
I used case 5 because outcrops of the aquifer show both horizontal and vertical fractures. Case 5 is depicted in Figure 1. The matrix blocks are mathematically treated as spheres. The spheres have a hydraulic conductivity of Km and specific storage Ssm. The head in the blocks can vary with dimensionless distance (ρ) from the center of the block, and with time in accordance with flow to the fissure. The block can be separated from the fissure by a thin skin which produces a resistance to flow from the block to the fissure.