DID PRODUCED BRINES FROM AN OIL FIELD CONTAMINATE A SHALLOW WATER WELL IN SOUTH TEXAS?

A State of Texas agency, after review of their lease incident file, suggested that a shallow water well in south Texas may have been contaminated with brines from the production activities of an oil company. Geochemical and geological evaluation is inconsistent with that suggestion, revealing that the water well is actually a point-source of contamination. Seventeen 6" in diameter auger holes were drilled to a depth of 75' on the lease. Two continuous, shallow, water-bearing, non-lithified, fine grained sands (aquifers) were encountered throughout the lease. The upper sand is an average of about 10.5' thick and the lower sand is an average of about 27.5' thick. Both aquifers are confined and separated by an average of about 18.5' of dense clay-rich mudstones. The lower part of the lower sand is apparently an abandoned meandering stream channel that was cut into the underlying mudstone. The water samples were analyzed for four cations: Ca, Mg, K, and Na; three anions: HCO₃, Cl, and SO₄; and conductance and pH. Geochemical evaluation of the water samples from the auger holes reveals two major geochemical anomalies: a) an elongated N-S-trending sinuous-type anomaly that extends across the lease and is associated with the lowest portion of the lower aquifer; and b) a localized point-source-type anomaly that is associated with the water well and superimposed on the elongated N-S anomaly. Neither anomaly had a production water geochemical signature. The elongated N-S anomaly is natural in origin or from a source up- gradient and off the lease. The point-source anomaly is consistent with acid-rich fluids having been placed into the water well. The nature and extent of the Cl anomaly associated with the water well suggests that muriatic acid (HCl) had been placed into the well. Pits and etchings on the concrete well pad demonstrate that acid was present at the well site. The absence of BTEX in the water well indicates that the source of the acid is not spent acid from the acidizing of an oil well, for example, but possibly spent acid from the acidizing of a fresh water well.