Widespread belief in the positive effects of Hel acid on sandstone reservoirpermeability needs to be challenged. Results from this study show a variety ofresponses in the Cardium Formation ranging from large improvements in liquidpermeability to destruction of good reservoir rock. Critical variables in thetype of response include lithology and diagenesis. In almost all samples, acidde-stabi11zed the fines. Fines migration caused mixed results in this studybecause fines can be flushed through the plug, enhancing permeability. Underreservoir conditions the effects on permeabi1ity would be more unanimouslydetrimental. Additionally, limited evidence for the formation of permeabilitydamaging iron hydroxide precipitates has been observed.
This study was undertaken to assess the effect of HCI acid on the permeabilityof Cardium Format ion sandstones and conglomerates. Variables in the study weresimplified and two procedures were used in order to isolate and interpret theacid-rock interaction. Study results should enable engineers to make betterdecisions on completions, and improve the ultimate amount of hydrocarbonsrecovered.
Acid is routinely used in several stages of completion in sandstone reservoirs.It has been noted (1) that ferric iron (Fe+3) can form an insolubleiron precipitate as pH rises in spent acid. Iron hydroxide starts precipitatingat a pH of about 2. Commonly, downhole equipment is considered the only sourceof iron, however an additional source may exist in iron-bearing minerals of thesandstones themselves. A range of acid-rock contacts occur in fieldapplications 2. In the near wellbore region, rock contact with many porevolumes of actively circulating, unspent acid cakes place. The region furthestfrom the wellbore would theoretically be exposed to fewer pore volumes of spentacid. The higher pH in the spent acid would result in a greater probability ofiron hydroxide precipitaticn in this region. The test program, described in thefollowing, was designed to measure permeabi11ty changes caused by acidtreatment at these two extremes.
Eight sandstone and eight conglomerate plugs were selected from wells in theFerrier, Wi11esden Green, Crossf1eld East and Ricinus, Cardium Fields. Thesamples were chosen to cover a range of porosity and permeability values, andto represent various percentages of siderite (iron-carbonate cement) endchlorite (iron-rich clay). Plugs were cut from cores ranging in age from 1960to 1986. Sample properties are summarized on Table I. As discussed below and asnoted in Table I, twelve of the plugs were used to test multiple-contacts withfull strength acid (phase I), and four, all from Willesden Green Field, testedextended contact with reacted acid. All sandstone samples were well-sorted, with less than 6% fines in the bulk fraction (XRD). All conglomerate sampleswere poorly sorted, with less than 8% fines in the bulk volume (XRD).
In the first phase of the study, samples were cleaned in toluene and methanol, then porosity and permeability to air were measured. Next, liquid permeabilityto a de-aerated non-damaging laboratory brine was established at reservoirtemperature and pressure.
