Acid Tunneling in Carbonate Rocks: A Full-Scale Experimental Study

Acid tunneling is an acid-jetting method for stimulating carbonate reservoirs. Several case histories from around the world were presented in the past showing optimistic post-stimulation production increases in openhole wells compared with conventional coiled-tubing (CT) acid jetting, matrix acidizing, and acid fracturing. However, many questions about the actual tunnel creation and tunneling efficiency are still not answered. In this paper, the results of an innovative full-scale research program involving water and acid jetting are reported for the first time. The tunnels are constructed through chemical reaction and mechanical erosion by pumping hydrochloric acid (HCl) through conventional CT and a bottomhole assembly (BHA) with jetting nozzles and two pressure-activated bending joints that control the tunnel-initiation directions. If the jetting speed is too high and the acid is not consumed in front of the BHA during the main tunneling process, then unspent acid flows toward the back of the BHA and creates main wellbore and tunnel enlargement with potential wormholes as fluid leaks off, lowering the tunneling-length efficiency. Full-scale water- and acid-jetting tests were performed on Indiana limestone cores with 2- to 4-md permeability and 12 to 14% porosity, sourced from the same supplier. Many field-realistic combinations of nozzle sizes, jetting speeds, and casing pressures were included in the testing program. The cores were 3.75 in. in diameter × 6 in. in length for the water tests and 12 in. in diameter × 18 in. in length for the tests with 15-wt% HCl acid. The jetting BHA was moved as the tunnels were constructed, at constant force on the nozzle mole, to minimize the nozzle standoff. Six acid tests were performed at the ambient temperature of 46°F and two at 97°F. The results from the acid tests show that the acid-tunneling efficiency, defined as the tunnel length divided by the acid volume, can be optimized by reducing the nozzle size and pump rate. The results from the water and acid tests with exactly the same parameters to match the actual CT operations in the field show that the tunnels are constructed mostly by chemical reaction and not by mechanical erosion. The acid-tunneling efficiencies obtained from the full-scale acid tests are superior to the average tunneling efficiency of more than 500 actual tunnels constructed during more than 100 acid-tunneling operations performed to date worldwide. Although the tunnel lengths and acid volumes for the actual tunnels constructed during the previous acid-tunneling operations were recorded by the service company performing those operations, little downhole temperature and formation characterization data were provided by the operators to the service company. Thus, the downhole-temperature and formation-characterization effects on the acid-tunneling efficiency for the previous field operations are unknown. In this paper, we describe the full-scale water- and acid-jetting tests on Indiana limestone cores. The major novelty of this test program consists of performing all measurements with casing pressure, unlike all previous water- and acid-jetting studies performed at atmospheric conditions and reported in the literature, which is closer to the field conditions during CT operations. The novel understanding of the combined effect of the nozzle size, pump rate, and casing pressure significantly improves the actual acid-tunneling efficiency.