• Nanoparticles based muds and conventional muds were investigated.
• Shale swelling was minimized by using graphene nanoplatelets.
• Rheological performance was improved by using graphene nanoplatelets.
• HPHT filtrate volume was minimized by using graphene nanoplatelets.
A. Aftaba,b, A.R. Ismaila, Z.H. Ibupotoc
aFaculty of Chemical and Energy Engineering, Universiti Teknologi Malaysia, Malaysia. bPetroleum and Natural Gas Engineering Department, Mehran UETSZAB, Sindh, Pakistan. cDepartment of Chemistry, Sindh University, Pakistan
Received 28 January 2016; revised 19 April 2016; Available online 26 May 2016
Five different drilling mud systems namely potassium chloride (KCl) as a basic mud, KCl/partial hydrolytic polyacrylamide (PHPA), KCl/graphene nanoplatelet (GNP), KCl/nanosilica and KCl/multi-walled carbon nano tube (MWCNT) were prepared and investigated for enhancement of rheological properties and shale inhibition. Nanoparticles were characterized in drilling mud using transmission electron microscope (TEM) analysis. Mineralogical analysis of shale was examined by X-ray diffraction (XRD). Five shale plugs were prepared using compactor cell for the determination of shale swelling. Shale swelling was determined using the linear swell meter (LSM) for 20 hours.
Results revealed that basic mud and KCl/polymer mud systems shows 30% and 24% change in shale volume. MWCNT, nanosilica and GNP were added separately in the KCl mud system. 0.1 ppb of each MWCNT and nanosilica showed 32% and 33% change in shale volume. However, when the shale was interacted with WBM containing 0.1 ppb of GNP, it was found that only 10% change in shale volume occurred. The results showed that the addition of nanoparticles in the KCl mud system improved the shale inhibition. API, HPHT filtrate loss volume, plastic viscosity (PV) and yield point (YP) were improved using GNP.
It is learned from the experimental work that small concentration of KCl with GNP can mitigate shale swelling compared to the mud contains higher concentration of KCl and PHPA in WBM. Thus, GNP can be a better choice for enhancement of WBM performance.
Drilling operation requires an extra care in well monitoring, rig hoisting, rig power, and most importantly well control system. Proper handling of well control system is only possible by well control equipment such as blow-out preventer and proper formulation of drilling muds , . Functions of drilling mud are to maintain the hydrostatic pressure when formation pressure exceeds the drilling mud pressure, to cool drill bit when drilling in hard geological formations for longer time, and to suspend and transport drilled cuttings from subsurface to surface. However, these functions can be well performed with the proper treatment of drilling muds rheology . Furthermore, rheological properties of drilling muds such as mud density, PV, apparent viscosity (AV), YP, gel strength, mud filtrate loss volume and lubricity are important to maintain for an efficient drilling operation and wellbore stability.
Shale causes world’s 70% of wellbore instability problems. Shale instability is caused due to presence of clay minerals into the shale. These clay minerals in particular kaolinite, smectite and montrolite have great affinity with the water . However, clay minerals start to swell after they interact with the water and as a result, clay swelling raised the wellbore instability such as shale sloughing, tight hole, caving and reduce efficiency of mud to lift the drilled cuttings.
Clay swelling reduces the rate of penetration (ROP) due to bit balling with sticky clay . Previously, Reid et al.  determined shale swelling behavior of north sea fields by interacting with different types of drilling muds. It was found that performance of tetra-potassium pyrophosphate (TKPP) was equivalent to OBM. However, TKPP muds shown mud accretion problems. Traditionally, KCl and PHPA are used to minimize the shale swelling problems. Somehow, KCl mud performance is good for shale swelling inhibition but the use of high concentration of KCl in drilling mud is strictly prohibited due to environmental concerns .
Beside that polymers such as acrylamide and PHPA are good heat insulators and used for prevention of mud filtrate, and inhibition of clay swelling . These polymers cannot sustain high pressure high temperature (HPHT) downhole conditions . Oil-based mud (OBM) and synthetic-based mud (SBM) are widely used for shale inhibition and considered as good drilling lubricants. OBM and SBM minimized the shale swelling because of less water content in their composition. Usage of OBM in environmental altered areas is considered to be illegitimate . There is no doubt that OBM came up with excellent shale inhibition properties, but it raised some operational problems such as it disturbed well logging data, and sometimes it raises formation damage , .
Therefore, oil and gas industry is more interested in WBM. It is used to drill almost 80% of all wells. It contains about 80% of water phase and 20% drilling additives. High water content drilling muds normally result in high friction and mud filtrate volume, low PV, and a great affinity with shale which leads to wellbore instability problems. Sehly et al.  found the way to minimize the concentration of KCl in WBM and reduced to environmental acceptable limit. Rodrigues et al.  used the multi functional polymers to modify rheological and shale inhibition properties of drilling muds. Moreover, Abdou et al.  evaluated Egyptian bentonite and nano bentonite as a drilling mud. It was found that use of local bentonite and nano bentonite is not suitable without using necessary drilling mud additives.
In this paper, experimental work has been conducted to minimize shale swelling and improve rheological performance of WBM using GNP, MWCNT and nanosilica in WBM. Behavior of nanoparticles in drilling muds was studied by TEM. Rheological and shale inhibition performance of nanoparticles based drilling muds is compared with conventional KCl and KCl/PHA mud systems.
The methodology discussed in this paper was based on the laboratory work. All the drilling mud testing work was carried out as per recommended practice API 13B-1 for investigating WBM .
Drilling mud additives such as KCl, caustic soda, flowzan, PAC, PHPA, and barite were provided by Scomi Oiltools. However, GNP, MWCNT, and nanosilica were purchased from Ugent tech Sdn Bhd.
Graphene nano platelets
GNP is colloidal suspended particles in aqueous solution produced by the chemical modification of graphite. GNP has multi functional properties and it consists of 3–8 layers (average thickness), diameter of particle ranging from sub micron to 50µm and surface area is 750 m2/g. It signifies the new class of nanocarbon. GNP can improve matrix material such as surface hardness, stiffness and strength. Such attributes of GNP fascinated to use for clay swelling inhibition and wellbore strengthening. Physical properties of GNP are shown in Table 1.
Table 1. Physical properties of GNP.
Multi-walled carbon nanotube
MWCNT has a unique structure. It could be metallic or semiconductor depends upon the diameter and chirality of tube. MWCNT is made up of tens of graphene sheets . Physical properties of the MWCNT are shown in Table 2.
Table 2. Physical properties of MWCNT.
Characterization of the materials
Nanosilica mud, GNP mud, and MWCNT mud were characterized using TEM. 0.1 g of each nanoparticles was separately added in 40 ml of diluted drilling muds and sonicated for 30 min before carrying out TEM analysis. TEM was conducted using biological-TEM Hitachi model no. HT7700. Shale was characterized using XRD. It was determined using Rigaku smart lab X-ray diffractometer R&D 100. XRD of shale is shown in Table 3.
Table 3. XRD of shale.
Preparation of drilling muds
Basic WBM was prepared by adding fresh water, barite, KCl, NaOH, flowzan, and PAC. The composition of muds, mixing time, and amount of drilling muds additives are given in Table 4.
Table 4. Formulation of drilling muds.
Preparation of homogeneous colloidal dispersion of nanoparticles
GNP, nanosilica and MWCNT were added at concentration of 0.1 ppb after barite in the mud formulation. Before adding nanoparticles in WBM, the nanoparticles was dispersed in SDS surfactant. 0.1 g of each nanoparticles were dispersed in the 50 ml of reagent bottle containing 20 ml distilled water and 0.1 ml of the surfactant. The reagent bottles were placed in an ultra sonicator for 30 min until homogeneous dispersion of nanoparticle in the solution can be seen.
Rheological properties such as PV, YP, 10-s gel strength (10-s GS), and 10-min gel strength (10-min GS) were determined using rheometer as shown in Fig. 1. API filtrate and HPHT filtrate volume were obtained using low pressure filter press and HPHT filtrate volume tester as shown in Fig. 2(a) and (b). Total five drilling muds were investigated for rheological and shale swelling behavior. Experimental conditions for the determination of rheological and shale swelling are given in Table 5. HPHT filtrate volume was found at 500 psi, 250ºf. PV and YP were calculated by Eqs. (1), (2).
Ф600=dial reading at 600 RPM, and Ф300=dial reading at 300RPM
Figure 1. Fann rheometer with Baroid hot cup.
Figure 2. (a) API filtrate volume tester, and (b) HPHT filtrate volume tester.
Table 5. Experimental conditions for the measurement of shale swelling, rheological properties, lubricity and API filtrate volume.
Coefficient of friction (CoF) was determined by using lubricity tester as shown in Fig. 3. Lubricity was calculated by using Eqs. (3), (4), (5), (6).
with instrument set at 60 RPM and pressure of 100 lbs, which are
whereas, CoF = Coefficient of friction, and CF = Coefficient factor
Figure 3. Ofite lubricity tester.
Linear shale swelling
Prior to start of drilling operation, it is very important to know the compatibility of drilling muds with the wellbore. The method to examine the compatibility of the shale swelling is to interact the shale with the drilling muds. In this study, shale plugs were prepared using compactor cell for swelling test. Procedure for determination of the shale swelling test is provided in linear swell meter (LSM) 2100 instructional manual . Experimental set up for shale LSM 2100 is shown in Fig. 4.
Figure 4. Fann linear swell meter.
Results and discussion
Typical TEM images for nanosilica in drilling mud are shown in Fig. 5(a) and (b) which indicates the particle nature of nanosilica and no damage in the morphology is observed in the drilling mud. The MWCNT seem to be thin porous in morphology and the shape is retained even in the use of drilling mud as shown by TEM study in Fig. 6(a) and (b). TEM images of GNP are provided in Fig. 7(a) and (b) which shows that clear view of graphene is described by TEM analysis.
Figure 5. (a) and (b) TEM images of nanosilica in drilling mud.