Various formation damage mechanisms occurring in the Cardium sandstone have been documented. Many of these have been studied in the context of injectivity and other secondary recovery techniques. Recently, increasing attention is being devoted to damage in this formation from a drilling perspective. In the past, drilling induced damage on vertical wells was not a significant concern since most completions involved hydraulic stimulation to enhance production. Now, as horizontal wells and slotted-liner completions become a viable option to operators attempting to increase reserves and production in tighter, less permeable zones, the damage-while-drilling issue becomes extremely important.
The objective of this paper is to present and discuss the methodology and testing results used in selecting a drilling fluid to be employed in drilling a horizontal well in the Cardium gas reservoir at Minehead in Central Alberta. Various aspects of both the design of the fluid and its implementation are presented. The initial production results are also included.
Some of the most difficult challenges in fluid design are perpetrated by tight, sub-irreducibly saturated, sandstone gas reservoirs. Although the task is less daunting when these reservoirs are to be drilled underbalanced, diligence is still required. Underbalanced fluid design must focus on both fluid-fluid and fluid-rock compatibility as a contingency against unplanned periods of overbalance or against the occurrence of spontaneous counter-current imbibition1.
When drilling overbalanced, the fluid design must incorporate one additional dimension - the selection of constituents to create an appropriate filter cake. This is to minimize the loss of whole fluid and mud filtrate into the reservoir. Because the very nature of cake deposition and "bridging" on pore throats and fractures denotes some degree of spurt loss associated invasion, the same design diligence must be applied to the liquid phase of the overbalanced drilling fluid. The solid phase of the fluid - the bridging particles, must be soluble in some fluid when it's time to produce the well. In sandstone reservoirs, even if pore throats are plugged with a portion of insoluble silicate drilled solids, if some soluble product is also present, bridge integrity will often be degraded so that production can begin.
The design of the bridging system should also be such that bridging occurs at the first pore throat so that effective "lift off" can also occur.
The problem is that the bridging materials available today are either oil soluble, acid-soluble or water-soluble. Often (not always) these solvents are incompatible with the sandstone gas reservoir, especially if there is clay in the pore throats or if the cementation matrix is carbonaceous or if the reservoir lacks water. The problem is complicated in that many horizontal wells in formations with questionable competency are completed using a slotted liner, making any type of selective stimulation almost impossible.
Various formation damage mechanisms occurring in the Cardium sandstone have been documented, most of them in the context of production enhancement. These include clay swelling, fines migration, paraffin deposition, scale and bacteria2. The design focus for this well centered on swelling clay and another mechanism - phase trapping or water blocking.