SOURCE AND COMPOSITION OF POTENTIAL END-MEMBER BRINES ASSOCIATED WITH SHALE GAS PRODUCTION IN THE APPALACHIAN BASIN OF CENTRAL PA

The Marcellus gas shale is an attractive target for unconventional gas exploration because of its high gas saturation (S_g ~0.8 - 0.9; S_w of ~0.1 - 0.2). The shale is dehydrated and contains no mobile water. When exposed to meteoric water during hydraulic fracturing the shale undergoes spontaneous osmosis and diffusion that imbibes most of the injected water. Only ~10-20 vol. % of the residual treatment water (RTW) is recovered and it tends to be highly saline, often with Total Dissolved Solid (TDS) contents exceeding 250 kppm. The Marcellus RTW is under-saturated with respect to halite.

When plotted in the ternary NaCl-H₂O-MCl₂ system (where M=Ca²⁺+ Mg²⁺+Ba²⁺+Sr²⁺), compositions of flow back brines from 55 Marcellus wells show a remarkably uniform composition of NaCl:MCl₂ of 57.8:42.2, indicating that their compositions are controlled by equilibration with the shale matrix. Most brine compositions in the Appalachian Basin are consistent with this bulk shale matrix composition indicating that they originated by dewatering of bounding shales.

We recognize two other end-member brines. One group plots to the NaCl side of the shale matrix line and the other group plots to the MCl₂ side. The former are high TDS brines that contain a component of Salina Salt, a Silurian-age evaporite (~70 wt.% halite, 15-20 wt.% anhydrite, 10 wt.% dolomite and 2-4 wt.% quartz). The latter group is a Ba-rich brine containing up to 15 kppm Ba and in central PA is sourced from the underlying Lower Devonian Oriskany sandstone. The Oriskany Fm. brine has reported TDS in excess of 300 kppm.

Our analysis shows that most published Oriskany brine compositions from the Appalachian Basin are actually mixtures of Oriskany Fm brine and Salina Salt. These brine mixtures all plot to the NaCl side of the shale matrix line. When Oriskany Fm brine is mixed with brine containing a Salina Salt component, the resulting brines are often saturated with respect to halite and depleted in sulfate due to precipitation of BaSO₄ as a result of Ba mixing with Salina derived sulfate. This is typical of most published Oriskany Fm brine data from West Virginia and Ohio. By contrast, mixtures of Oriskany Fm brines with Marcellus RTW are saturated with respect to both halite and/or solid barium chloride dihydrate (BaCl₂.2H₂O). These mixtures plot to the MCl₂ side of the shale matrix line.