The specific objective of this research is to ascertain the high gravity solids and low gravity solids that gives better hole cleaning during oil and gas drilling operations while the main objective is to measure the effect of mud weight in achieving effective hole cleaning of the wellbore.

The transportation of cuttings and good hole cleaning is very important in any drilling operation because a successful drilling operation is vital to a profitable business in the oil and gas sector. A successful drilling program is a product of an efficiently cleaned hole whereas, a poorly cleaned hole implies the accumulation of formation cuttings in the well [1]. This leads to a number of problems which include; Stuck pipe, increased cost of drilling, decreased rate of penetration, formation fracturing and many others. The basic function of a drilling fluid is the transportation of cuttings generated during drilling activities and this must be done effectively, with the hole being cleaned with a minimum accumulation of cuttings in the annulus. This will, therefore, prevent downtime during drilling operations and the costs will therefore, be optimized [2]. The ability of the operations to effectively remove drill cuttings dictates the efficiency of a drilling operation. Hence, the efficiency of the cutting removal and hole tidiness is a very vital requirement factor for meeting the desired drilling objective. Cuttings transport efficiency of a drilling program depends on a number of factors such as cuttings concentration, rate of penetration, fluid flow regime, hole geometry and inclination, and rheology amongst others. It is necessary to carry out analysis on drilling mud to determine their rheological properties, transport efficiency and all other factors that are necessary for a good hole cleaning operation [3].

Adari et al [4], arranged the parameters controlling cuttings transport in their order of relevance and from their illustration, fluid rheology, volumetric flow rate and rate of penetration have the highest control on the efficiency of hole cleaning. In horizontal wells, cuttings removal is easier with turbulent flow than laminar flow. As the angle of inclination of the well increases, the circulation time required to clean the wellbore increases [5]. Drilling cuttings, under the influence of gravity, tend to fall through the ascending drilling fluid. The velocity at which this tends to occur is known as the slip velocity. The fluid velocity must be high enough to overcome the slip velocity of the particles for effective cuttings removal [6]. A proper hole cleaning exercise is the efficient and effective transport of drilled solids from the wellbore to the surface. This helps with the reasonable unhindered movement of the tubular and drill strings [7]. A good understanding of the mechanics of cuttings transport is important for the optimization of drilling hydraulics for a hitch-free drilling [8].

Demirdal [9] compared theoretical frictional pressure losses with experimental data. Hydraulic diameter approach was employed for the Bingham Plastic Model, Power Law model and Yield Power Law model [10]. The final analysis indicated that calculated pressure losses overestimate the measured values in both laminar and turbulent flow conditions. Osei [11] noted that adequate cuttings removal from a well while drilling is critical for cost effective drilling, as high annular cuttings build up often leads to high risk of stuck pipe, reduced rate of penetration and other impediments to standard drilling and completion. The investigation was on the rheological parameters that influence the removal of cuttings in non-vertical boreholes. Micah [12] reiterated the importance of accurate estimation of annular pressure losses in drilling and well completion operations, and calculated the pressure loss gradients/ Equivalent Circulating Density (ECD) using annular frictional pressure equations as the basis of relating the results obtained from the rheological model- equivalent diameter definition combinations.

Unegbu [13] highlighted the various factors that affect the cuttings removal and made an attempt to develop a model to bridge the gap between existing models, of which a single model was proposed, and a chart was also developed. Jachnik [14] reported that flow properties are important for understanding hole cleaning and suspension characteristics. Iyoho and Zamora [15] noted that an increase in mud rheology can reduce ECD if the hole cleaning efficiency is simultaneously increased. Ochoa [16] carried out a study for selecting the rheological model that best fits the rheological properties of a given non-Newtonian fluid. Igwilo [17] noted that the most relevant function of a drilling fluid is the ability to carry drill cuttings from the bit up to the annulus to the surface. The study further explained that the rheology of the drilling fluids and the associated annular hydraulics is directly dependent on the hole cleaning efficiency; and canvassed for adequate understanding of rheology as essential for cost effectiveness in meeting the drilling objective. Also, Okarjni and Azar [18] carried out a study on the effects of field-measured mud rheological properties on cuttings transport. They observed that the annular mud velocity in vertical well drilling must be sufficient to avoid cuttings settling and assist in transporting these cuttings to the surface in a reasonable time. Azar and Sanchez [19], in their study, made an interesting review on the factors that affect cuttings transport. They highlighted drilling fluid rheology, drill string rotation, drilling rates, flow rate and annular eccentricity, among others. The density of drilling mud has an associated effect on carrying capacity [20].

This section is divided into three parts, namely: (a) Mud formulation (b) Mud weights and Rheological properties measurements (c) Evaluation of annular pressure drop, cutting concentration and transport efficiency.

Based on the Modified power law principle, Pressure loss is given by:

(1)

Where, V_a = Annular velocity in ft/sec

d_1, d₂= Hydraulic diameter, inches

Other related formulas from i to vii [21]:

(2)

Where V_a= Annular velocity in ft/sec

q = Flow Rate in gpm

(3)

Where γ_p= Shear rate in s^-1

k_a= Annular consistency index

n_a= Annular flow behaviour index

(4)

Where τ_p= Shear stress

T = Cutting thickness in inches.

(5)

Where γ_p = Shear rate in s^-1.

V_s = Cuttings slip velocity in ft/s.

d_p = Diameter of the drill pipe in inches.

(6)

Where T_r= Cuttings Transport Ratio

(7)

Where T_c= Cuttings Transport Efficiency

(8)

Where C_a = Cuttings Concentration in vol. %

ROP = Rate of Penetration in ft/hr.

The results from the calculations of annular pressure drop, cuttings transport efficiency and cutting concentration are presented in Fig. (1). A good hole cleaning procedure practically depends on the type of weighting material used and the practice applied during the drilling operation. This analysis involves the use of barite and calcium carbonate as weighting materials. The fresh mud was prepared using both weighting materials. Fig. (1) shows the effect of the weighting materials on annular pressure drop. A sample of barite 4.2 SG and calcium carbonate 2.7 SG was used to carry out the laboratory tests. Both weighting materials from the plots gave good results but barite gave a better result. From the plots, at a mud weight of 8.7ppg, calcium carbonate gave a pressure drop of 11.037psi while barite gave a lower pressure drop of 10.052 psi. This trend continued for both weighting materials at subsequent mud weights. Barite gave a low annular pressure drop which would result in a better ECD and better hole cleaning operation.

Fig. (2) shows the effect of the weighting materials on transport efficiency in shale and sandstone formations. Calcium carbonate gave a higher transport efficiency result compared to barite in both formations. This result is backed up by the findings of Kippax and Ward-Smith (2013) who explained that calcium carbonate has finer particles compared to barite after careful analysis using a master sizer 2000 from Malvern instruments and a laser diffraction analyser. They further explained that fine particles are usually associated with high viscosities that promote the suspension and transportation of cuttings. This accounts for the high transport efficiency given by calcium carbonate in both shale and sandstone formations.

Fig. (1). Effect of weighting materials on annular pressure drop.

Fig.(2). Effect of weighting materials on transport efficiency in shale and sandstone formations.

Fig. (3). Effect of weighting materials on cutting concentration in shale and sandstone formations.

It is also evident from Fig. (2) that shale formation has higher transport efficiency than sandstone formation with both weighting materials. Cuttings size slightly influences hole cleaning. According to the work of Larsen et al (1993), small particles are more difficult to clean than larger particles. This is because smaller particles form more compact beddings compared to bigger particles that form loose compact beddings which can be easily swept away. Sandstone has more loose and unconsolidated particles compared to shale. Shale has a larger cutting size this is why it gives a better transport efficiency.

Fig. (3) shows the effect of weighting materials on cutting concentration in shale and sandstone formations. From the plots, calcium carbonate gave a better result than barite in both formations. A low cutting concentration is ideal for a good hole cleaning procedure. This is because of the high transport efficiency already portrayed by Calcium carbonate because of its fine particles as observed by Kippax and Ward-Smith (2013) so there tends to be lesser amount of drill cuttings when calcium carbonate is incorporated into the drilling mud than when barite is used. As a result of this, calcium carbonate is applicable in depleted reservoirs (low pressure reservoirs) and it can act as a bridging agent. It also minimizes formation damage.

It is also evident from Fig. (3) that shale gives a lower and better cutting concentration using both weighting materials compared to sandstone. The higher the transport efficiency, as already explained above from Fig. (2), the lower the cutting concentration. High cutting concentration would lead to high ECD and higher chances of hole cleaning issues. The summary of the selection criteria based on the analysed results for both weighting materials is shown in Table 9.

The following conclusions were derived from this study:

1. Drilling mud weighted up with barite gave a better annular pressure drop than the drilling mud weighted up with calcium carbonate.
2. Calcium carbonate gave a better result in terms of cutting concentration compared to barite in both shale and sandstone formations.
3. Calcium carbonate gave a better cutting transport efficiency compared to barite in both shale and sandstone formations.
4. Shale gave a better transport efficiency and cutting concentration for both weighting materials.
5. The higher density weighting material gave a better hole cleaning result than the lower density weighting material