Polyoxyalkyleneamine as shale inhibitor in water-based drilling fluids

fluids may cause swelling of the clay minerals

in shale formations that can create major problems for the drilling

operations. According to statistics (

Dzialowski et al., 1993), shales

account for about 75% of the drilled sections in oil and gas wells, and

cause approximately 90% of the wellbore instability-related problems

during drilling operations. When water-sensitive shales are exposed

to the conventional water-based drilling

fluids, shales have an

immediate tendency to take up water from the drilling

fluid.

Depending on the chemical characteristics of the shale, this can result

in a rapid swelling or dispersion of the shale. Consequently, typical

problems such as bit-balling, disintegration of cuttings, borehole

wash-out, high torque and drag, and stuck pipe are often encountered

as a result of water adsorption by water-sensitive shales (

Steiger and

Leung,1992; van Oort et al., 1994

). During the drilling practice in shale

formations, various chemicals have been used to reduce clay mineral

or shale swelling in water-based drilling

fluids. Among the chemicals,

salts such as potassium chloride, sodium chloride and divalent brines

are the earliest and most widely used for inhibition of water-sensitive

shales. The inhibitive method relies on the use of high concentration

of salts. These salts somehow retard the hydration and swelling of

water-sensitive shales through a variety of mechanism. However,

these salts in large quantities adversely affect the environment. These

salts also

flocculate the clay minerals resulting in both high fluid losses

and an almost complete loss of thixotropy. Further, increasing salinity

often decreases the functional characteristics of drilling

fluid

additives. In the

field application, polymer/KCl drilling fluids became

popular (

Clark et al., 1976; Twynam et al., 1994). A variety of polymers

in combination with KCl were evaluated to achieve a higher level of

shale inhibition as compared to KCl alone. However, surface swelling

due to insuf

ficient shale inhibition and high viscosity of polymers

offered little partial success in providing the satisfactory results.

Since drilling operations impact the surrounding environment, the

drilling

fluid additives should have low toxicity levels and should be

easy to handle and to use to minimize the dangers of environmental

pollution and harm to operators. It is desirable that additives work

both onshore and offshore. As an excellent inhibitive agent, it should

control the swelling of the clay minerals and drilled formations

without adversely affecting the desired performance of other

additives and the rheological properties of drilling

fluids. In this

paper, we evaluated the toxicity, inhibitive property and compatibility

of polyoxyalkyleneamine as a shale inhibitive agent in water-based

drilling

fluids.

2. Experimental

2.1. Materials and reagents

Encapsulating agent (UltraCap) and low viscous polyanionic

cellulose (PAC-LV) were supplied by M-I SWACO in America. Modi

fied

starch, xanthan gum (XC), cationic inhibitor (CSW-1), carboxymethyl

starch (CMS), viscosi

fier (80A51), filtration control agent (CMJ-2) and

borehole-stabilizing agent (JYW-1) were all available in domestic

market. Sodium-montmorillonite (Na-MMT) was made in Huai'an in

Hebei in China. Polyoxyalkyleneamine (POAM) was synthesized

according to the references (

Rasshofer, 1985; Gbard, 1985; Larkin

and Renken, 1988

).

2.2. Rheological determination

The rheological properties of the

fluid samples in this study were

measured using a model ZNN-D6 viscometer. The properties included

apparent viscosity, plastic viscosity, yield point and gel strength. The

apparent viscosity, plastic viscosity and yield point were calculated

from 300 and 600 rpm readings using following formulas from API

Recommended Practice of Standard procedure for

field testing drilling

fl

uids (Recommended Practice, 1988):

Apparent viscosity (AV)=

ϕ600/2 (cP)

Plastic viscosity (PV)=

ϕ600−ϕ300 (cP)

Yield point (YP)=

ϕ600−AV (N/m2).

2.3. Na-MMT and cuttings dispersion test

The inhibitive agents in the mud systems should be suf

ficient to

reduce the hydration and swelling of clay minerals in the shale

cuttings. By a trial and error method of testing the combination of

fresh water (or drilling

fluids) and clay in the shale cuttings, the

approximate amount of POAM present in the experimental systems

was determinated and the concentration of POAM was about 2 wt.% of

the systems. To demonstrate the inhibitive property of POAM, the

experiments were designed to determine the maximum amount of

Na-MMT that can be inhibited by 2 wt.% POAM solution over several

days. All samples were adjusted to about pH 9 and treated with a

2.5 wt.% portion of Na-MMT at a medium rate of shear. After stirring

for 30 min, the samples were hot rolled in a roller oven for 16 h at

70 °C. When the samples were cooled, their rheologies were recorded.

All samples were treated again with Na-MMT as previously described.

This procedure was carried out for each sample until they were too

thick to measure.

Before cuttings dispersion tests, the crystalline components of the

cuttings were carried out by using Rigaku TTR-III X-ray diffractometer.

Cuttings dispersion tests were performed by hot rolling 30 g shale

cuttings (6-10 mesh) in 300 mL different drilling muds for 16 h at

100 °C. Before hot rolling, all muds were adjusted to about pH 9. After

hot rolling, the remaining cuttings were screened using a 40 mesh

screen andwashed with 10 wt.% KCl solution, dried and then weighted

to obtain the percentage recovered of the cuttings in the different mud

systems.

2.4. Biological toxicity and compatibility test

The biological toxicity of the shale inhibition agent was measured

by the Mysid shrimp test. A detail account of the procedure for

measuring toxicity of drilling

fluids is described in the reference (Duke

et al., 1984

).

The compatibility test was carried out by measuring the change of

rheological properties and

filtrate loss of several mud systems before

and after adding the shale inhibitive agent. Filtrate loss of drilling

fl

uids was determinated by a filter press. The filtrate loss of the mud

systems was measured in milliliters under 690 kPa of pressure

through a special

filter paper for 30 min.

3. Results and discussion

3.1. Effect of POAM on Na-MMT dispersion

Rheological properties were measured for the different systems

after adding Na-MMT (

Table 1). The plots of apparent viscosity (AV),

plastic viscosity (PV) and yield point (YP) of the samples versus Na-

MMTconcentration were shown in

Fig. 1–3 respectively. MMT in fresh

water easily absorbed water and swelled greatly (Fig

. 4). As a result,

the viscosity of the system signi

ficantly increased, and the systemwas

too thick to measure the rheological readings after adding 12.5 wt.%

Na-MMT in fresh water. However, the other three systems exhibited

the inhibitive property at different degree (

Table 2).

Compared with fresh water, 2 wt.% KCl solution and 2 wt.% CSW

solution, the 2 wt.% POAM solution exhibited the superior performance

to inhibit the hydration and swelling of Na-MMT. Despite KCl

can reduce the swelling and hydration of most clay minerals,

especially smectite, it is only moderately effective for illite and may

actually increase swelling of kaolins (

van Oort, 1997; Santarelli and

Carminati, 1995

). As a kind of low molecular mass polyamines, the

reaction of POAM with clay minerals can involve several mechanisms

including hydrogen bonding, dipole interactions and ion exchange

(

Theng, 1974; Sithole and Guy, 1985). The molecules of POAM were

adsorbed on the surfaces of clay minerals and compete with water

molecules for reactive sites, which could serve to reduce clay mineral

swelling. Low molecular mass amines can also be intercalated.

3.2. Effect of POAM on shale cuttings dispersion

Experimental cuttings were obtained from the shale formation of

Chang-301 well depth of 1550 m in Yumen Oil

field. The samples were

composed mainly of clay minerals, feldspar and quartz. The shale

cuttings studied had a high clay fraction. Generally, shales with a high

clay content should easily absorb water molecules and swell greatly.

Shale cuttings were a clay-rich waste stream generated by the

drilling process and could become unstable by the mechanisms

similar to the hydration and swelling of clay minerals. The results of

the shale cuttings dispersion tests (

Table 3) indicated that the

recovery rate of the shale cuttings increased after adding 2 wt.%

POAM in three different mud systems. Especially for the former two

mud systems, the recovery rate of the shale cuttings increased from

67.0% to 89.0% and from 72.3% to 90.3%, respectively. Since the shale

cuttings had a high clay mineral content, POAM exhibited a good

inhibiting property and reduced the swelling or hydration of shale

cuttings effectively.

3.3. Biological toxicity and compatibility of POAM

According to the reference (

Duke et al., 1984), the biological

toxicity of POAMwas measured by the Mysid shrimp test and the LC50

of POAM was much greater than 30000 ppm. Generally, an LC50 value

of greater than 30000 had been considered as environmental

compatibility. As a shale inhibitive agent in water-based drilling

fl

uids, POAM exhibited low toxicity and could be used both onshore

and offshore.

The results of the compatibility test were shown in

Table 4. POAM

had no in

fluence on the rheological properties and filtrate loss of the

mud systems before and after adding the shale inhibitive agent. This

indicated that POAM did not change the performance of other

additives and the rheological properties of drilling

fluids. Thus,

POAM could be compatible with the conventional additives in waterbased

drilling

fluids.

4. Conclusions

A new shale inhibitive agent in water-based drilling

fluids was

described. POAM was an excellent shale inhibitive agent. Na-MMT

dispersion tests, gel strength and relative viscosity measurements

suggested that the addition of Na-MMT had little in

fluence on the

rheological properties of the POAM solution. POAM exhibited superior

performance to inhibit the hydration and swelling of Na-MMT. In

cuttings dispersion test, the recovery rate of the shale cuttings

increased after adding 2 wt.% POAM in several water-based drilling

fl

uids. The test indicated that POAM could suppress the hydration and

swelling of shales effectively. The determination of toxicity and

compatibility of POAM showed that POAM was environment-friendly

and compatible with other conventional drilling

fluid additives.

Acknowledgements

Wewould like to thank the

financial support from PetroChina Fund