Relating shale mineralogy and microstructure to swelling activity: using Na-bentonite/sand pellets as a synthetic shale

Abstract

Shale formation instability presents significant challenges to oil and gas operations. Clay minerals within shale may react with water-based fluids (WBFs) causing swelling, aggregation of shale cuttings on the drill string (bit balling) and potentially wellbore collapse. Factors affecting clay hydration downhole still require investigation.

The swelling response of shale was studied to understand swelling-mechanisms, and potential causes, that lead to shale instability. The relationship between mineralogical composition, compaction pressure and formation age were investigated and related to shale-fluid interaction mechanisms within a wellbore region.

Expansive clay minerals typically associated with swelling-related shale instability are smectite or mixed-layer clays. Wyoming bentonite (montmorillonite-clay) has high absorption and swelling rates when hydrated, and for this reason, was used as an analogue for rich sub-sea shale formations. Artificial shale cores were made by varying quartz content (0-30%) and quartz size fractions (125-250 μm, >250-300 μm and >300 μm). Sample preparation conditions were varied with two compaction pressures (44.25 MPa, 88.50 MPa) and three compaction-time (5, 8 and 10 minutes) intervals.

Clay swelling experiments frequently involve monitoring the one-dimensional displacement with time of an initially dry clay core as it imbibes water from a supply at its base. While KCl is predominantly used in drilling and completion fluids to reduce the hydration of shales and thereby swelling-related instability, the purpose of these novel experiments was to ascertain how mineralogy impacts swelling of shale. Therefore, NaCl brine (0.5 and 2 mol/L concentrations) was selected for the electrolyte to induce lattice expansion within the clay.

The data implies that increasing burial depth (i.e. pressure increase) has a less significant effect upon swelling and instability than previously thought. Instead, the age of the sediment (i.e. compaction time) appears to have the most significant impact upon the structure of the rock, and thereby, the swelling. Other variables that were found to have a significant effect on the maximum swelling of an artificial core were quartz (%) content and saline concentration (>0.5 mol/L).

Some formations with water-sensitive clays do not always exhibit swelling behaviour, and it can be challenging to predict the response of these rocks to aqueous fluids downhole. These results indicate that dispersing capabilities, cementation, bedding orientation (i.e. diagenetic history) and pore water chemistry are all significant variables when predicting or testing for shale stability.