ABSTRACT

A well from northeastern British Columbia, which was drilled into the Cadotte Member (Lower Cretaceous), provides a good example of the significant effect that drilling fluids can have on the authigenic and detrital components of a reservoir, and on its subsequent production.

The fine-grained, relatively homogeneous Cadotte sandstones are composed primarily of quartz and chert. The principal authigenic components are siderite, calcite, chlorite and mixed-layer illite-smectite. The sandstones have good porosity and permeability with maximum values of 27 per cent and 1186 mD respectively. Petrophysical analysis of the Cadotte sandstone indicated the presence of gas in the upper 4 m but a subsequent drill-stem test of this interval failed to recover any hydrocarbons.

Detailed petrographic studies showed that the primary porosity of the reservoir was reduced mainly by mechanical compaction and precipitation of authigenic grain-coating chlorite and mixed-layer illite-smectite. The introduction of gas into the upper 4 m of the Cadotte Member effectively terminated diagenesis. In the lower, water-bearing sandstones, however, diagenesis continued with the growth of pore-lining and pore-filling clays.

To determine the relative effects of drilling and completion fluids on the reservoir, simple experiments were carried out to show the loss of permeability due to clay hydration and to show subsequent permeability improvement with an acid treatment. Although not trying to duplicate actual down-hole conditions, these simple laboratory experiments do reinforce the importance of ensuring the compatibility of fluids introduced into the well with the mineralogical components of a reservoir rock.