(79913)_Understanding_Formation_(In)Stability_During_Cementi(3)

钻井固井

SPE/IADC 79913 a washout in excess of 15 in. The lithology across this section (approximately 100 ft) had no appreciable change as indicated by gamma ray and resistivity logs. This formation was known to be composed of water-sensitive shales. The washouts were initially assumed to be caused by high flow velocities during the cement job and drilling damage during the subsequent drillout of the squeeze jobs. However, a review of the cementing reports did not reveal any excessive velocities or other rational explanation for the severe washouts. Consequently, the conventional assumption that the washouts were mechanically induced became suspect.

Recent work by Gdanski investigates the effects of various concentrations of high-pH solutions with and without KCl and NaCl on clays.5 While this work is focused on formation damage during stimulation treatments, and as such, is deficient in the manner described previously insofar as extending the interpretation of regain permeability testing to the understanding of shales, the basics of clay chemistry interactions with these types of fluids would be expected to be more or less consistent with that of cementing fluids. This line of reasoning is also consistent with prior literature on the subject of shale and fresh water cement slurry interactions. Not yet fully realizing the fluid/shale physicochemical effects, a trial-and-error approach was used at this point, based on combining the findings from Gdanski, conventional shale swelling tests (shown in Table 1), and field observations thus far. In this area and other well locations in the coastal Gulf of Mexico being scrutinized for problematic LOT/FIT issues, various concentrations of salts were added to the cementing spacers and slurries. The intention of the salts being to at least eliminate the potential detrimental effects of fresh water. Figs. 2A and 2B provide a similar comparison on an immediate offset well to the logs shown in Figs. 1A and 1B. No changes were made in the mud or cementing programs between these two wells, except for the addition of 4% KCl to all spacers and cementing fluids on the offset well. The severe washout was no longer present, and this offset was the first well in which the operator obtained a successful LOT without squeezing. Several such cases have since been documented with similar results. Although these cases may seem to present only circumstantial evidence, when coupled with the tests presented in the remainder of this paper, they provide a strong argument for adjusting the chemistry of cementing fluids for formation stability.

Fundamental Driving Forces and Elevation of Near-Wellbore Pore Pressure

The mechanisms that influence fluid (water) transport are differences between shale and wellbore fluid hydraulic pressure, chemical potential, electrical potential, and temperature.

The interaction process and the mechanisms of transport for shales exposed to water-based fluids can be quite different and complex. The molar free energies of all the constituents within the shale and the water-based fluid provide the driving forces that result in the transfer of water, cations, anions, etc. The sum of all the flow phenomena mentioned can result in a net flow. Equilibrium conditions will be dictated by the sum of these driving forces, which can generally be described by the relationship

3

Jv = k/µ ( Ph- Po)

Where

- Jv = net flow - k permeability - µ = viscosity

- Ph = overbalance driving force

- Po = sum of other driving forces such as osmotic

and thermal

The difficulty is in the mathematical treatment of these coupled physicochemical interactions between the water-based fluid and shale. Several investigators have used the nonequilibrium thermodynamic approach in the treatment of the transport process in shales.3,4 This approach allows the incorporation of cross effects between different phenomena, such as flux of a solution with different ionic species caused by the hydraulic gradients and/or chemical potential gradient of that species, as well as thermal and electrical potential.

In most cases, however, the two most relevant mechanisms for water transport in and out of shale are: (1) the hydraulic pressure difference between the wellbore pressure (equivalent total fluid column density) and the shale pore pressure, and (2) the chemical potential difference, i.e., water activity between the wellbore fluid and the shale.

Osmotic Semipermeable Membrane

The fine pore size and negative charge of clay on pore surfaces cause argillaceous materials to exhibit membrane behavior. The efficiency is a measure of the capacity of the membrane to sustain osmotic pressure between the wellbore fluid and shale formation. A mathematical representation to describe the driving force for the movement of water by an

osmotic (flow) mechanism is shown in Equation 1. =

RT Awdf V ×ln =±(σ× Pp)=± Aσ×(P Pp)……………(1) wsh

Where

- Awdf = water activity of the wellbore fluid that can be

estimated by various means, most notably, partial vapor pressure determination, boiling point elevation, or directly using a hygrometer

- Awsh = water activity of the shale pore fluid that can be

measured (for preserved shale cores) by partial vapor pressure determination - Pp = far-field pore pressure - P = near-wellbore pore pressure

- σ = the membrane efficiency term, specific to a shale-

drilling fluid or cementing fluid system

The concept of reflection coefficient6 i.e., membrane ideality, has been previously introduced to handle “leaky systems”, including WBM-shale systems used for borehole stability applications.3 For water-based fluid/shale systems, membrane efficiency (reflection coefficient) is not a clearly defined term (unlike an oil film on shales pr …… 此处隐藏：3753字，全部文档内容请下载后查看。喜欢就下载吧 ……