In general, wiper trips can be short or long for cleaning purpose or for making the wall more smooth and stable. A short trip is an action or some operation for tripping out / or in the drill string to a certain planned depth inside the open hole section. Performing a short trip helps to remove the cutting bed and to improve the smoothness of the wall. Sometimes a short trip can be done when there is a long sliding section via a mud motor. It is beneficial to do so because sliding with the mud motor creates a lot of cutting beds that are not effectively removed.

A long wiper trip is a similar action as a short trip, but the trip is longer. Typically, the drill string is pulled out from the open section and then is tripped back in the hole to the previous depth. This type of wiper trip can remove a lot of cutting beds. However, these trips take additional rig time. Not only time is spent, but also there is the risk of creating other problems as wellbore instability, formation damage, etc. Hole condition, torque, drag and field experience will dictate whether you need to do either a short trip or long wiper trip.

Drilling fluid is one of the most important elements of any drilling operation for hydrocarbon exploration [1]. Penetrating different types of formations need very close attention to the design of the drilling fluid for minimizing drilling problems and cost. Drilling fluid is the only system in the well construction that keeps us in continuous contact with the wellbore. The extent to which drilling mud properties must be controlled varies with the geologic condition [2]. A properly designed drilling fluid performs several essential functions during its circulation from the surface to the bottom of the well and up again to the surface [3]. The most important functions are to clean the well from the drilled cuttings and maintain wellbore stability. Wellbore instability may be due to chemical reactions of the drilling fluid with the drilled formations or due to mechanical issues. Many studies have been done on both the chemical and mechanical factors separately and in combination . Chenevert has studied mechanical properties of shale after hydration since the 1960s [4]. There are also many studies combining both of these factors [4-6].

Through controlling the wall stability, many operational problems toward more optimized drilling can be eliminated. In general, the primary objective of any cost control program is to maintain a low daily estimate of total expenditures for the entire drilling operation [7]. Cost of the drilling fluid ranges between10% to 20% of the total cost of a well drilling operation. For that, drilling fluid performance can affect overall well construction costs in several ways through the wellbore instability which is the largest source of well problems [3].

At the beginning of the 1950s, many soil mechanic experts were interested in the shale swelling, which are important for maintaining wellbore stability during drilling, especially in water-sensitive formations. It is reported that shale accounts 75% of all formations drilled by the oil and gas industry, and 90% of wellbore stability problems occur in shale formations [5, 6], [8, 9]. The first and most element in controlling wellbore instability is the drilling fluid passing through the (i) transport drill, cutting of the hole and separation of cuttings from the drilling fluids on the surface, (ii) formation a thin filter cake on the walls of the wellbore and preventing the inflow of drilling fluids into the formations and (iii) inhibiting the inflow of formation fluids into the wellbore [10]. Due to the swelling problems, a bridge, pack-off and tight spots will accrue. Several works focused on the selection of drilling fluid for a specific clay formation [11-13]. More recent studies on shale - fluid interactions suggest a new approach to Water-Based Muds (WBM) designs [14, 15]. Consideration is given to maintain borehole stabilization in reactive shale by reducing hydration (swelling) and/or clay dispersion. Most of the laboratory works were also done on analyzing the effect of additives on different types of drilling muds. Besides all the advances and developments in the rheological properties, still, there are some operational works like wiper trips that need more consideration and later analysis. In general, the purpose of wiper trips and reaming is to give smoother and clean well before casing running. Wiper trips and reaming can be combined with the use of the most suitable drilling mud [16]. Wiper trips can also increase the reactivity of shale formations by disturbing filter cake situation in the previously drilled intervals.

In Kurdistan, most of the well instabilities accrue in shale formations. Formations of Kolosh, Aliji and Tanjiro are the most problematic formations in drilled wells. Problems of clay dispersion or shale swelling that lead to well collapse, tight spots and pack-off are dominant problems of chemical reactions of drilling fluid with penetrated formations. Well instability for the other formations beneath theses like Shiranish, Kometan and Qamchuqa is due to mechanical instability. Wiper trips can give positive results in some formations which mostly contain limestone and negative results in other shale formations.

Daily drilling operations and most wiper trips in clay formations for more than 26 wells drilled in Taq Taq oil field, Bazian block and Meran block in Kurdistan, north of Iraq have been collected. Shale formations were the main sources of well drilling operation problems. Section 12 ¼´´ of well Bazian no. 1 (Bn-1) was selected as a key interval to be monitored closely for this study. The operation reports of drilling, mud circulating system, fluid additives, casing and cementing operations data were all collected for the 12 ¼´´ section. Following of all the wellbore problems and recording drilling parameters during wiper trips, monitoring the effect of drilling fluids additives effects on the rheological properties and its relation to wellbore stability was conducted . Data obtained from drilling and geological daily reports were analyzed to find out the effect and role of the changes in mud rheological properties. Two different calculation methods have been implemented for calculating the net rising velocity of the drilled cutting. Analyses of the cutting and coring samples were performed in Koya University laboratory for the rheological effect in the drilled section.

Kurdistan region in Iraq lies within the northern part of the Zagros Folded Belt and is estimated to contain about 45 billion barrels (bbls) of Iraq’s 115 billion barrels of oil reserves, making Iraq the sixth largest oil reserve in the world [17, 18]. Tectonically, the area of this study is complex and located in the unstable platform of the Arabian plate [18]. The studied key well Bn-1 is located at the border between Low Folded Zone and High Folded Zone [19]. Bn-1 is located in the middle of three oil blocks Taq Taq, Miran and Bazian. 26 oil wells have been investigated for the effect of wiper trips operations. The Bn-1 well pad is located 1082 m above sea level on the edge of the Bazian mountain ridge [20] (Fig. 1).

Fig. (1). Studied location.

Net rising calculations are used here to verify and check the validation of the wiper trips operations before the casing-running. For that, if the cutting increased with the wiper trip operations, this will be a strong indicator for other problems that cannot be solved with the wiper trips [24]. In this case, rapid improvement of parameters like flow rate, annular velocity, pipe rotation, ROP, mud weight (density), mud rheology and cutting size/shape should be performed. These parameters directly affect the standpipe pressure and frictional pressure down the hole [25]. The first and most effective parameter in this relation is the mud density. Through the mud density, there will be a high possibility to return pressure balance between the formation pressure and hydrostatic pressure and thereby, prevent settling more cutting down the hole. To find out the effect of wiper trips in Bn-1 on improving the drilling operations, we will start with the calculation of cutting slip velocity [26].

In general, there are two methods to calculate cutting slip velocity [27]. If the net rising velocity is positive, it means that the drilling fluid properties and the flow rate are with accepted values to carry out cutting to the surface. In this case, the wiper trips will leave a positive effect on the operations. On the other hand, if net rise velocity is negative, it means that the flow rate and fluid properties are NOT accepted to carry out the cuttings [28]. In this case, all the effects with wiper trips operations without any other improvement are useless.

The first method depends mostly on the real field data using equations (1 - 3) below:

The annular velocity Av is provided by equation (1):

(1)

Where, Av is an annular velocity in ft/min. Q is the flow rate or Triplex Pump Output in gpm (gallon per minute), Dh is the diameter of the hole in inch and Dp is the diameter of drill pipe in inch.

The cutting slip velocity Vs is given in equation (2):

(2)

Where Vs is the cutting slip velocity in ft/min, Pv is the plastic viscosity in centipoise, Mw is mud weight in ppg., Ds is the diameter of cutting in inch and Mws is cutting density in ppg.

The net rise velocity Nv is given by equation (3):

(3)

The general formula for getting Q in units of gpm is given in equation (4):

(4)

Where, SPM is Stroke per minute, R is the liner size (inch) and Ls is the stroke length (inch).

Calculation of the pump output Q in gpm before and after modification of mud properties gives 441.9 gpm and 368.2 gpm, respectively by using equation (4), where a Ls=6.5 inch and SPM=90 before and SPM=75 after modification. Input data for the equations above are given in Table 4.

The net rising velocity is estimated to Nv = 55 ft/min and 38.67 ft/min before and after mud modification, respectively. The increase in hydrostatic pressure by changing the mud properties prevents wall collapse and increases wall stability at the same time as a positive velocity is maintained. Thus, wiper trip was not necessary.

The second method for calculating the net rising velocity is quite different from the first method [27]. However, it is still a straight forward calculation. The cutting slip viscosity in centipoise is given by equation (5):

(5)

Where n is the power law exponent given by equation (6)

(6)

Where, θ600 is a value at 600 viscometer dial reading and is a value at 300 viscometer dial reading.

The fluid consistency K is given by equation (7)

(7)

The annular velocity Av is calculated by equation no. (1) and the slip velocity (Vs) in ft/min from equation (8):

(8)

The net rise velocity is again calculated from equation (3) above. Input data for the equations above are given in Table 5.

The net rising velocity is estimated to Nv = 76.3 ft/min and 63.7 ft/min before and after mud modification, respectively. The estimated flow rate is good for hole cleaning because the annular velocity is more than cutting slip velocity.

Koya University laboratories were used in preparing many WBM-polymer drilling fluids with different densities, pH and other rheological parameters. High Pressure - High Temperature (HPHT) filtration tests were performed and a Viscosity-Gel meter (VG) was used to find rheological properties based on the Bingham-Plastic model equations.

• The drilled 12 1/4'' section can be divided into two parts. The upper part is composed of Kolosh and Aliji formations that contain a high percentage of clay and the lower part is mostly limestone of the Shiranish formation with a low percentage of clay and this was indicated by previous researchers like Saad Z. Jassim and Jeremy C. Goff also [29].
• Hole problems in the upper part are mainly caused by chemical interactions between drilling fluid and a high percentage of clay like problems of tight spots and bridges. The problems in the lower part are mostly mechanical instabilities due to the high percentage of limestone which can be solved through the increase of mud density. Problems like well pack-off and bridge were more dominant in this part [23].
• Casing setting point of surface section locates inside the Aliji formation making the formation of two parts and a weak setting point as been shown in Fig. (11).
• Mud logging records unit and cutting sampling matched the lithological description. The lithological description of the penetrated formations Aliji, Tanjero and Shiranish is presented in Table (6). The Aliji formation contains a higher percentage of clay than Tanjero and Shiranish. The last two formations are composed mainly of fractured marly limestone with some clay beds.

Fig. (9). Part of MLU data of Bn-1 oil well [30].

• All the field and laboratory tests enabled us to realize that the density between 11-12 ppg can give a very good over-balanced drilling operation in drilling the upper part of 12 ¼´´ section and 14-15 ppg for the lower part.
• Polymers like PAC and PHPA optimize mud rheological properties. Table 7 and Fig. (10) show the laboratory results of Equivalent Mud Density (EMD) for each of the penetrated formations in the 12 ¼´´ section.

• Drilling performance reduced due to long time and high cost consumed for the wiper trips operations towards solving the well instability [30].
• Instability of shale formation caused the stuck of BHA, downhole tools fishing, loss of equipment and side tracking operations.
• High chemical reactivity of the shale resulted in poor hole logging, inability to the land casing on the planned setting point and poor cementing conditions/jobs. Essentially increase in nonproductive time and increase in total drilling cost.

Fig. (10). Bn- Lab calculated pressure and equivalent mud weight EMW with depth.

Fig. (11). Drilled sections in Bn-1 Side Track 1

• Little over balanced mud property can sustain well stability in the upper part of 12 ¼´´ section where the chemical reactions are the main causes of drilling hole problems.
• Based on the positive results of the calculated net rising velocity, the lower part of the section is tectonically stressed and composed mainly of limestone. Wiper trips in tectonically stressed sections will be the waste of time if we do not return the over balance situation.
• Wiper trips in the upper part of 8 ½´´ section will leave a positive effect and negative effect on the lower part of the section.
• PHPA (partially hydrolyzed polyacrylamide) improves rheological properties like viscosity and gel strength and makes a thin gelatin barrier covering borehole wall. These improvements decrease the invasion of the formation and clay dispersion also.
• Changing the drilling program for the surface section in Fig. (11) to drill and case all Aliji formation with the surface section will help to overcome many challenges and difficulty in the intermediate section.
• Wiper trips with homogenous improve in drilling fluid properties can give positive effects in solving problems in the 12 ¼´´section.