Low-density water-based drilling fluids formulated with hollow glass spheres (HGS) offer an attractive drilling method. HGS are incompressible lightweight additives with the capability to reduce mud weight down to 41.0 lbm/ft3 (5.5 lbm/gal). Several pressure ratings of HGS are available, and selecting the appropriate rating is essential to avoid formation damage and lost circulation in near-balance conditions. Failure of the spheres could thus lead to catastrophic results.
The objectives of this paper is to evaluate the stability of inhibited water-based drilling fluids formulated with HGS in diverse pH environments, and assess their potential application in Wasia formations in Saudi Arabia. Wasia formation is composed of middle cretaceous clastic rocks with layers of sandstone, shale and occasional limestone. Wasia is an aquifer with a thick unit that crops out in central Najd with a slight eastward dip (Powers et al. 1966). The pressures of Wasia correlate to equivalent mud weights (EMW) of 51—58 lbm/ft3. This range of EMW is lower than that of water, and the use of conventional mud systems could introduce several operational difficulties due to the high hydrostatic pressure they create. The implementation of HGS-based lightweight fluids would lower the hydrostatic pressure in the wellbore, thus eliminating or reducing the frequent loss of circulation experienced in Wasia. HGS-based fluids could also provide greater protection of underground water resources, improve rate of penetration (ROP), and reduce or eliminate differential pipe sticking.
We conducted comprehensive analysis of HGS performance in various pH environments to assess their stability in drilling fluids. Mud characteristics and rheological properties were examined before hot rolling (BHR) and after hot rolling (AHR) to determine the effects of high pressure and high temperature (HPHT) on the system. The duration at which the samples were exposed to HPHT conditions varied from 1 to 4 days to understand the behavior of the mud over time. In addition, two typical formulations were prepared using HGS and conventional additives to evaluate their properties and compare them to the American Petroleum Institute (API) standards.
Hollow glass microspheres were found to be stable in the pH conditions of drilling operations (pH ~9), with a maximum density variation of 0.5 lbm/ft3. At higher pH levels (pH >11), the spheres experienced fractional dissolution due to the reaction of the added NaOH with borosilicate glass. In pH ranges lower than 4, the spheres were found to be extremely stable. The inhibited water-based fluids formulated with HGS produced favorable rheology and stable mud characteristics before and after exposure to the actual downhole temperatures of Wasia formation.