A universal fluid (UF) is typically a water-base fluid that has been treated with finely ground blast furnace slag and that still maintains the appropriate characteristics of a good drilling fluid. The slag becomes concentrated in the filter cake formed while drilling permeable formations and slowly sets to form a hard layer intimately bonded to the formation. True zonal isolation can he obtained by using a UF and subsequently cementing with slag-based mud solidification technology. Complete mud displacement and efforts to remove filter cake are not necessary prior to cementing since the solidified UF filter cake bonds strongly both to the formation and to the cement and since undisplaced portions of the UF set up as well.
The universal fluid (UF) has boon developed primarily out of the need to improve cementing in horizontal and extended-reach wells. To date UFs have been used to improve zonal isolation and to reduce or prevent lost circulation or cement fallback during drilling and cementing. Two Diatomite wells in the Belridge Field, California and the vertical portion of one horizontal well in the Midway Sunset Field, California were successfully drilled with a UF and cemented with slag mix cement slurries. Also, a well in the Midway Sunset Field, where losses are routinely experienced, was drilled with a UF specifically to control lost circulation, and no losses were experienced. Five wells in the Peace River area in Canada were successfully drilled with a UF to prevent cement fallback upon cementing.
A UF is a drilling fluid with the appropriate characteristics of a drilling fluid (i.e., rheology control, carrying capacity, fluid loss control, filter cake quality, density, lubricity, etc.) which contains a hydraulic material, which forms a filter cake and is a fluid which can be activated to set. This paper reports on UFs which use blast furnace slag (hereafter referred to as slag) as the hydraulic material. Other common hydraulic materials such as Portland cement, flyash, natural pozzolanic materials, etc. may be used in conjunction with slag or as substitutes for slag.
The development of UF technology was facilitated by slag's unique characteristics:
it is inexpensive and widely available;
it is a latent hydraulic material, i.e, it is basically an inert solid until activated;
it is therefore compatible with water-base drilling fluids (in contrast to Portland cement);
common mud-thinning chemicals act as retarders; and
simple alkaline materials such as caustic soda and soda ash are excellent activators.
Use of a UF for drilling a well provides two primary benefits:
a settable filter cake is deposited on any permeable portions of the wellbore, and
any undisplaced pockets of the UF which are left after cementing will, with time. set up at downhole conditions.
If the well is cemented using slag-based mud solidification technology (hereafter referred to as slag mix),1–4 two additional benefits are obtained:
the UF, UF filter cake, and the slag mix are completely compatible and intimate bonding is formed among the materials; and
after the slag mix is in place. the activators in the slag mix will migrate or diffuse into both the filter cake and undisplaced UF, causing an accelerated setting.