Page 252 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

236 Gas Wettability of Reservoir Rock Surfaces with Porous Media

While using oil-based drilling fluids, wetting agents are added to alter water-
wet cuttings into oil-wet cuttings in order to avoid accumulation of water on
the surface of water-wet rock cuttings, which causes sedimentation and
increase in viscosity. When the cuttings and wetting agents interact, the interfa-
cial tension between cuttings and base oil decreases greatly. The interfacial
energy is very low in the system, having thermodynamic stability. At the same
time, as the viscosity of external phase is strong, clay and other solid grains are
adsorbed on the oil and water interface, forming a thin layer and making it
very strong and dynamically stable. There are, however, big challenges in
degreasing oily cuttings.

Oil contained in oily cuttings is distributed in the form of free oils, oils adsorbed
on the cutting surfaces, and oils adsorbed on the cutting pores. Oils contained in
oily cuttings are base oil and wetting agents and other treatment agents. Oils in
the present state are free oils, water, and solid-phase grains in oily cuttings that
form emulsions. Free oils are external phases. Oil adsorbed on the surface of cut-
tings includes both the surfactant adsorbed on the cutting surfaces and part of
base oil, mutually dissolved with the oil-wet end of surfactant. The pore oil
within cuttings is regarded as universal in nature. Due to the capillary force, it is
very difficult to remove pore oils using a chemical method. A better approach is
to grind the cuttings to micron size, and then the pore oil can be extracted.
Correspondingly, the basic idea of determining degreasing of oily cuttings is:
a. weakening the viscosity of external phases, and removing oils in the outer
layer, while making use of emulsification; b. destroying the solid-liquid inter-
face formed between the wetting agents and cuttings, and removing adsorbed
oils; and c. altering the cutting surface to make it more gas wettable in order
to increase the capability of desorbing oil and making sure that the oil-
desorbed cuttings don’t adsorb oils anymore. In view of the high viscosity of
external phase emulsion in oily cuttings, and the presence of high-value metal
ions contained in the oil-base drilling liquid, some auxiliary agents (such as
emulsifier, water softener, etc.) are usually added to assist degreasing during
the drilling process.

In all, the design principle for the new and efficient degradable oily cutting is:
1. Possess gas-wetting alteration capabilities. The cutting surface can be
altered into a gas-wetting surface to realize hydro-oleophobicity and
enhance the degreasing effect. This is the core of the new technology.
2. Possess emulsifying properties to form oil-in-water type droplets.
3. Possess permeation capabilities to adsorb strongly on oil and water
interface and promote emulsification, while stripping off oils from the
adsorbed surface.
4. Possess wettability alteration capability to strip oils on the surface of
solid-phase grains through a reversible adsorption displacement.
Based on the above principles, a new and efficient degradable oily cuttings for-
mula has been designed. It can be used for washing the oil base and oily