Friday, May 4, 2018

Designing and Managing Drilling Fluid (2)

Shale instability is largely driven by changes in stress and chemical alteration caused by the infiltration of mud filtrate containing water. Over the years, ways have been sought to limit interaction between mud filtrate and water-sensitive formations. So, for example, in the late 1960s, studies of mud-shale reactions resulted in the introduction of a water-base mud (WBM) that combines potassium chloride (KCl) with a polymer called partially-hydrolyzed polyacrylamide-KCl-PHPA mud. PHPA helps stabilize shale by coating it with a protective layer of polymer-the role of KCl will be discussed later.

The introduction of KCl-PHPA mud reduced the frequency and severity of shale instability problems so that deviated wells in highly water-reactive formations could be drilled, although often still at a high cost and with considerable difficulty. Since then, there have been numerous variations on this theme as well as other types of WBM aimed at inhibiting shale. 

However, in the 1970s, the industry turned increasingly towards oil-base mud (OBM) as a means of controlling reactive shale.  Today, OBM not only provides excellent wellbore stability but also good lubrication, temperature stability, a reduced risk of diffrential sticking and low formation damage potential. OBM has been invaluable in the economic development of many oil and gas reserves.

 The use of OBM would probably have continued to expand through the late 1980s and into the 1990s but for the realization that, even with low-toxicity mineral base-oil, the disposal of OBM cuttings can have a lasting environmental impact. In many areas this awareness led to legislation prohibiting or limiting the discharge of these wastes.

To develop alternative nontoxic muds that match the performance of OBM requires an understanding of the reactions that occur between complex, often poorly characterized mud systems and equally complex, highly variable shale formations.

Requisites for a Successful Drilling Fluid

Most OBM is an invert emulsion comprising droplets of aqueous fluid surrounded by oil, which forms the continuous phase. A layer of surfactant on the surface of the water droplet acts like a semipermeable membrane, separating the aqueous solution in the mud from the formation and its water. Water will pass through this membrane from the solution with the lowest concentration of a salt to one with the highest-osmosis.

A key method of maintaining shale stability using OBM is to ensure that the ionic concentration of the salts in the aqueous-internal-phase of the mud is sufficiently high, so that the chemical potential of the water in the mud is equal or lower than that of the formation water in the shale. When both solutions have the same chemical potential, water will not move, leaving the shale unchanged. If the water in the internal phase of the mud has a lower chemical potential than the fluid in the formation, water will travel from the shale to the mud, drying out the rock. Unless dehydration is excessive, this drying out usually leaves the wellbore in good condition.

In WBM, there have been many efforts to protect a water-sensitive formation from mud filtrate. One technique is to introduce a buffer in the form of blocking and plastering agents, ranging from starches and celluloses, through polyacrylamides to asphalts and gilsonites. Total control cannot be achieved in this way so specific inhibiting cations- chiefly potassium and calcium ions -are traditionally added to the base water to inhibit the clay from dispersing-to stop it from breaking up when attacked by aqueous solution. This is achieved by providing cation exchange with the clays in the shale the K+ or Ca2+ commonly replace the sodium ion (Na+) associated with the clay in the shale, creating a more stable rock that is better able to resist hydration. Hence KCl-PHPA fluids.

The movement of WBM filtrate from the wellbore into the surrounding shale is controlled by the difference between the chemical potentials of the various species in the mud, and the corresponding chemical potentials within the formation. Chemical potential depends both on the mud's hydrostatic pressure in the wellbore and on its chemical composition.

To design an effective WBM, it is necessary to know the relative importance of mud differential pressure versus chemical concentration and composition, and how this relates to the type of mud and formation. For example, if the rock is chemically inert to WBM filtrate (as is the case with sandstone) then invasion is controlled solely by the differences between the wellbore pressure and the pore pressure within the rock. But for shale, opinion varies. Some experimenters suggest that the shale itself can act as a semipermeable membrane, making the chemical components the key determinant.

A number of relatively new types of mud systems have been introduced. For example, one route is to substitue the oil phase in OBM with synthetic chemicals. In this way, the excellent characteristics of OBM may be reproduced with a more rapidly biodegraded continous phase than was available before. 

 Typical synthetic base chemicals include esters, ethers, polyalphaolefins, linear olefins and linear akly benzenes. One of the chief disadvantages of these systems is that they tend to be relatively expensive compared to conventional OBM. However, such systems can still be cost-effective options compared to WBM. 


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