# Casing Wear Series – 2: Prediction

1. Wear Mechanism

The casing wear model applied in CWPRO (casing wear prediction software developed by PVI) assumes that the metal volume worn away in a wear groove section is proportional to the frictional energy transmitted to the casing by a rotating tool joint as shown in Figure 2 in the Casing Wear Series – 1: Causes.

The transmitted frictional energy is defined in this formula:

Where:

E  = Frictional Energy, lb-ft

μ  = Friction factor, dimensionless

SF= Side force on tool joint per foot, lbf/ft

SD = Sliding distance traveled by the tool joint against casing wall, in

The volume of casing wall removed per foot in time t hours is mathematically expressed in the equation:

Where:

WV = Casing wear volume per foot, in3/ft

WF = Wear factor, E-10psi

SFdp = Side force on drill pipe per foot, lbf/ft

Dtj = Tool joint OD, in

N = Rotary speed, rpm

t = Rotating time, hr

The definition of wear factor is the ratio of friction factor to specific energy, which is the amount of energy required to remove a unit of steel. The unit for wear factor is E-10psi-1; therefore, when a wear factor is reported as 8, the actual value used in casing wear calculation is 8E-10psi-1.

Quite a few experiments were conducted to find the casing wear factors under different mud systems, tool joint materials, casing interior and drill string protectors. Among them, Maurer Engineering Inc. conducted joint-industry project DEA-42. It was reported that more than 300 laboratory tests were performed under DEA-42 to determine the wear factors for various drilling conditions.

For a typical water-based mud, WF can vary as follows:

Normal or low: 3 – 7

Medium: 8 – 13

High: 14 – 20

WF above 20 can be considered as very high and may cause severe casing damage.

2. Wear Geometry

A typical wear groove is shown in the following figure.

Figure 1. Casing Wear Groove

The relationship between wear depth and casing wear volume is:

Where:

WV = casing wear volume per foot, in3/ft

h = wear depth, in

r = tool joint outer radius, in

R = casing inner radius, in

S = R - (r - h), in

P = (R + r + S) / 2, in

3. Software

Based on the R & D results from the past two decades, PVI developed CWPRO, software that enables us to understand the casing wear phenomenon and accurately predict casing wear before the drilling operation or perform a post-drilling analysis.

CWPRO is a comprehensive casing wear prediction software with built-in torque and drag function. For every incremental drilling interval, the amount of energy transferred from drill pipe to casing is calculated. Accumulative wear and wear depth are first obtained and then the burst and collapse strength of the worn casing can be assessed.

Being a time-dependent incident, casing wear deepens as we drill deeper. Figure 2 shows the sequence of drilling and corresponding wear profile along the previously set casing.

Figure 2. Time-dependent Casing Wear

# “All that is gold does not glitter” - Drilling Mud

ScienceNewsforKids.org has recently posted an article titled “Mud worth than gold”, which happened to be the “Nose in the News” project for my 3rd grade son. Though this article is a little hard for a 3rd grader to analyze, its title is rather attractive. The article tells the sample mud collected from far below Antarctica’s ice by two scientists and their drilling crew contains valuable information. This might help reveal the secrets of the continent’s ancient climate, thus help on future weather prediction.

Coincidentally, in this September, a report from “marketsandmarkets.com” also has something to say about mud. The report with a long title "Drilling Fluids (Drilling Mud) Market and Completion Fluids Market: by Types (Water-Based Systems, Oil-Based Systems, Synthetic-Based Systems, Other Based Systems), Application Areas (Onshore and Offshore), & Geography - Global trends and forecast to 2018 " defines and segments the global drilling fluids and completion fluids market with analysis and revenue forecast. It predicts that the drilling fluids and completion fluids market will grow from an estimated \$10.6 billion in 2013 to \$15.2 billion by 2018.

Regardless of its meaning and the context, the title of J. R. Tolkien’s poem “All that is gold does not glitter” would certainly apply to drilling mud. According to industry statistics, drilling mud takes approximately 10% to 15% of the total drilling cost. With the needs of more deep sea drilling and ever fast drilling rate, drilling fluids are used on day-to-day basis and plays a vital role in drilling process: controlling formation pressure, sealing permeable formations, stabilizing the wellbore, suspending the drill cuttings, and cooling and lubricating the drilling bit etc.

Drilling fluids are basically categorized into 3 types: water-based mud (WBM), oil-based mud (OBM) and synthetic-based fluid (SBM). When choosing a drilling fluid, factors like well design, cost, technical performance, environmental impact all need to be considered. WBM and OBM are commonly used nowadays. They are complex compositions with various additives such as minerals and chemicals. As the well drilling reaches various depths, it requires different type of drilling fluids to meet the specific drilling condition, where demands the balance between the properties and the additives in the mud. Small problems with mud may lead to severe problems like lost in mud circulation, gas escaping or even blowout.

Among those who work on the drilling rigs, mud engineers are the ones who frequently and closely deal with the drilling mud. Their job is 24/7.  A mud engineer’s duty not only involves in prescribing mud treatments, maintaining the drilling fluids, but also keeping continuous mud reporting every day. To them, time management is more important than ever. Besides powerful computer aids, software like MUDPRO developed by PVI is a great way to enhance the mud engineer’s ability on mud data recording and analyzing, hydraulics calculating, inventory tracking and daily reports/recap generating.

As drilling technology advances, drilling fluids are innovated and designed to be not only cost effective but also environmentally safe. Research on Non-toxic bio degradable drilling fluids with nanotechnology is being conducted and huge investment is being made. This type of fluids not only provides higher transfer efficiency and better thermal conductivity but also removes toxic metals. After all, protecting the environment has enormous impact to our future generations.