Slag/mud mixtures improve cementing operations in China

Dahua Wu, Peiyan, Bozong Huang
Chinese National Petroleum Corp.
Tanggu, China

In many cases, the slag/mud slurry outperforms conventional Portland cements in oil and gas wells.

Mud conversion to cement technology has been studied for half a century and has attracted the attention of the drilling and cementing industry worldwide, because of its economic, technical, and environmental benefits.^1-3 Particularly since slag-mix technology was developed by Shell Oil Co. in 1991, it has been successfully used on more than 163 cementing jobs including primary, temporary abandonment, and sidetrack plug cements.^{4 5}

The use of slag-mix has been somewhat controversial. Some experts claim slag-mix is the most important progress to date in mud-to-cement conversion and has become another choice for cementing practices.⁶ They also believed there were no fundamental limitations to its application downhole, and conceivably the material could be used for any well cemented. Other experts have different points of view and thought it might have limits for oil field use.⁷ In their studies, the basic mud had to be diluted by 60% or more with water before the blast furnace slag (BFS) was added. Their slag slurries showed a high incidence of cracking and apparent brittle nature, bad settling stability, and volume shrinkage.

To date, the Chinese National Petroleum Corp. (CNPC) has used mud solidification by slag successfully on 22 cementing jobs in the Sichun, Changqing, Jidong, and Shengli oil fields. The major purpose of these investigations was to determine the application of slag-mix technology to various cementing operations.

Blast furnace slag

The chemical composition, character, and other information on BFS have been reported in the literature.^{4 8-9} The BFS used in this study was the byproduct of China Capital Steel & Iron Co. (Table 1 [31159 bytes]).

Before the lab testing and field application, the slag had to be dried and ground to the desired fineness. A special activator system, BA-1 and BA-2, was adopted instead of soda ash and caustic. As a result, the BFS's fineness of 230 sq m/kg, instead of 400-650 sq m/kg, was sufficient for the slag and mud mixture to set and form cement with enough compressive strength.

Table 1 [31159 bytes] also lists the chemical composition of BFS discharged from 41 steel and iron plants in China in 1990. The total BFS production exceeds 30 million metric tons per year in China. BFS comes from more than 21 provinces, covering a greater area than that by oil well cement producers. The transportation costs for BFS is almost half that for conventional oil well cement.

The average quality coefficient Ko is 1.67. Ko is the oxides content ratio of BFS and represents the hydraulic ability of BFS:

Ko = (CaO + Al2O3 + MgO)/(SiO2 + MnO + TiO2)

The standard deviation is low, 0.149. BFS with Ko more than 1.4 makes up about 95% of the total produced, which is suitable for oil well cementing. Therefore, BFS is an available and cheap cementing material in comparison with oil well cement.

Blast furnace slag is a latent hydraulic material, which can be hardened by chemical activators or thermal energy. The chemical activators include alkali metal oxides and hydroxides, or Portland cement, combined with various salts and can control the setting time of slag/mud mixture. In general, a caustic, soda ash, and lime system is adopted, but they lead to severe flocculation of the mud system (Table 2 [45357 bytes]). This flocculation is the reason why the mud must often be diluted by water to accommodate the BFS. This heavy dilution results in little environmental impact from a BFS/mud mixture.⁷

The BA-1 and BA-2 system, a complex metal hydroxide, provides the minimum readings of Fann-35 viscometer of modified mud and BFS slurries, and the maximum 24-hr compressive strength development (keeping the same activator concentration with Na2CO3 and NaOH). So the dilution of mud has not been needed usually in CNPC's mud solidification experiences, unless the drilling fluid contained a high concentration of active clay. Obviously, the selection of the activator system is critical to the environmental benefits of slag-mix technology.

Mud solidification

From the Dagang and Bohai oil fields, three typical field muds were taken from various points in the drilling process and were converted into cement slurries by adding BFS, activators, and other chemical additives. The muds were a 1,160 kg/cu m freshwater/bentonite/hydrolyzed polyacrylonitrile (HPAN) polymer spud mud used for the surface hole, a 1,135 kg/cu m fresh water/

bentonite/polyacrylamide (PAM) and carboxymethyl cellulose (CMC) polymer mud used at the intermediate casing point, and a 1,920 kg/cu m sea water/KCl polymer mud used at the final stages of a well (Table 3 [90056 bytes]).

The BFS/mud mixtures were prepared according to the following procedure:

1. Combine drilling mud and required dilution water, and mix. This process is needed only for high-density mud systems (Formulation 3) because of its high concentration of active clay and drilling solids.

2. Mix activator BA-1 and BA-2, dispersant, and retarder for 2 hr or more to make the additives uniformly dissolve in the modified mud.

3. Slowly add BFS during a 1-min period at low mixing speed, then continue mixing for another 35 sec at high speed.

4. Pour the BFS slurries into rheometer cups, molds, and other test devices to determine the BFS slurries' properties according to American Petroleum Institute (API) recommended procedures.10 Because no cracking occurs in the tested BFS slurries under atmospheric or pressurized conditions, the strength development was measured with API standard crush tests.

BFS slurry thickening time can be tailored to satisfy the various downhole temperature demands by controlling the concentration of activator, dispersant, and thinner (also a retarder). The thickening time curve improves when the consistency can be developed from 30 BC (Bearden unit of slurry consistency) to 100 BC in less than 30 min. (The slurries often hardened in the cup before the chamber was cooled for disassembly.) The quick thickening time is beneficial for wells in which gas migration is a concern, because gas invasion often occurs at the transition period of liquid to solid.

Strength tests were performed on samples cured at 20° C. higher or lower than bottom hole circulating temperature. Table 3 [90056 bytes] also shows that BFS can be set to form enough compressive strength over a wide temperature range.

One advantage of this system, compared with conventional Portland cement, is that it can eliminate two-stage cementing operations and save material costs and rig time.⁹

The fluid-loss results indicate that BFS/mud mixtures also meet the demand of various cementing jobs on fluid loss control. It depends upon the fluid loss of the solidified mud and compatibility of mud fluid loss controller with BFS.

Settling stability

Settling stability has become an essential property for cementing fluids, especially for slurries designed and used in deviated and horizontal well bores, because unstable cement slurries or slag mixtures cannot provide effective zonal isolation.

The procedure to evaluate the settling stability is to measure API free water content.¹⁰ Free fluid is neglected because it cannot set to prevent or inhibit communication of fluid between the various formations penetrated by the well, and downhole temperature and pressure are excluded. This procedure is not a convincing measure of stability.

In 1990, BP Exploration developed a new method to test slurry stability by measuring the density segregation of setting cement. This method combined aspects of temperature, pressure, and deviation in a unique chamber to simulate downhole conditions.¹¹ Settling particles often occur in low-density cement slurries, slurries with dispersants or retarders, and high-density slurries with weighting agents.

The finished BFS slurries of Formulations 1 and 3 in Table 3 [90056 bytes] were poured into a modified BP apparatus (20-cm cement column), placed and cured in a pressurized chamber for 24 hr at 20° C. higher than BHCT, and then disassembled and cut into seven sections to determine every section density.¹² In this test, the volume shrinkage is also able to be measured if it exists.

Fig. 1 [54931 bytes] shows that volume shrinkage and particle settling barely occur in BFS slurries. Columns V and VI had little or no shrinkage, and there was only slight density variance between the top and bottom sections. In contrast, the Portland cement-based slurries (I-IV) had shrinkage and density variations. Note that the density variance between the top and bottom in sample VI is only 7 kg/cu m, even though its density is as high as 2,160 kg/cu m. It is difficult to formulate a conventional Portland cement and weighting agent to ensure the same settling stability as that of the slag mixture.

Fracture toughness

All set cements, including Portland cement, fiber cement, slag cement, and slag/mud mixtures, are intrinsically brittle materials. Their difference lies in fracture toughness, which is the resistance to crack propagation.

Excessive brittle cementing material will affect perforation performance. Fracture toughness of set cement can be evaluated by stress-strain curves and critical energy strength factor (Eic). CNPC has performed a great number of stress-strain tests on cement, latex-cement, and fiber cement. Eic is defined as the mechanical energy that specific volume samples (50.8 3 50.8 3 50.8 mm, except the shrinkage samples) can bear divided by compressive strength, while the samples are destroyed. The greater the Eic, the more acceptable the toughness of material.

Similarly, the flatter the stress-strain curve, the more acceptable the cement. These tests were conducted at a 100 kN/min loading rate. Table 4 [44516 bytes] shows that Eic of the slag mixture is between Class G cement and fiber cement. Its stress-strain curve is also sharper than that for fiber cement but flatter than that of Class G cement.

Slag or cement mixed with mud can provide more Eic than that of cement mixed with water. This is likely because the plastic, hydrated clay links slag or cement particles to improve the resistance to crack propagation. This also helps explain the higher compressive strength and improved volume stability for slag or cement/mud mixtures.

Microstructure

A scanning electron microscope (SEM) was used to examine the neat slag or cement slurry, slag mixture, and cement/mud mixture. These samples were prepared in the same manner and formulation as those in Table 4 [44516 bytes], but they were cured for 8 days.

The neat slag slurry had a network of interconnected particles, and fiber-like hydration products were discovered on the surface of slag particles. The slag mixture had an integrated structure composed of closely accumulated plates without holes between the plates. This resulted because the clay participated in structure formation by filling up the space between slag particles.

Class G cement had images similar to those of neat slag slurry, but it was composed of irregular particles. The hydration products were like needles formed on the surface, and more holes were observed than in the slag. No visible differences were discovered between the SEM images of the cement/mud mixture and neat cement slurry. The cement flocculated the hydrated clay.

These microstructure examinations helped explain the results from some of the tests above. Moreover, the SEM photos helped explain the lower permeability of slag mixture than cement.¹³

Strength retrogression

Both slag and cement can hydrate to form a gelatinous calcium silicate hydrate called C-S-H gel.¹⁴ At temperatures above 110° C., C-S-H gel is subject to metamorphosis and often converts to a phase called alpha dicalcium silicate hydrate (a-C2SH), which usually results in decreased compressible strength and increased permeability of the set cement. This phenomenon is known as strength retrogression.

The strength retrogression problem can be prevented by reducing the bulk time to silica ratio (C/S) in the cement slurry. Silica flour or fine silica sand is typically added to Portland cement slurry by 30-45% (by weight of cement), to reduce the C/S ratio to 1.0 or more preferably to 0.6 for higher curing temperatures. The C-S-H gel gradually forms minimal deterioration minerals known as tobermorite (C5S7H), xonotlite (C6S6H), gyrolite (C6S7H2), and truscotlite (C7S12H3), instead of converting to a-C2SH while the curing temperature is increased from 110 to 250° C.¹⁵

The C/S ratio of slag is less than that of Portland cement (determined from the chemical composition), so the BFS slurries will provide more temperature stability than cement. To make the BFS slurries suitable for deep oil and gas wells, geothermal wells, and thermal recovery well cementing jobs, the following two questions must be answered: