# Circulation Sub Series—3: Case Study Part I of II

Case Study

Engineers may have some basic ideas on how to optimize the design parameters of a circulation sub to achieve their goals. For example, increasing the total flow area of a circulation sub will increase the bypass flow rate, reduce pump pressure, etc. This case study will quantify the impacts of various circulation sub parameters and fluid properties on pump pressure and ECD for a wellbore cleanup operation. We used a wellbore cleanup hydraulics software to perform this case study. Numerical methods are employed to obtain the correct flow split percentage at the location of the circulation sub. The flow split is obtained such that the summation of the frictional pressure losses inside the pipe below the circulation sub and in the annulus below the circulation sub should be equal to the pressure loss through the circulation sub nozzles.

Figure 2 shows the wellbore configuration used for the example calculation. This is the basic case, from which we will perform sensitivity studies on each of 5 parameters. Note that the flow rate is left blank because it is run at several values for all stages.

Figure2: Example Case

Figure3: Flow Paths

1. Effect of Total Flow Area (TFA)

Circulation sub’s adjustable nozzles enable you to define how the flow is split between the annulus and the pipe interiors. By adjusting the TFA of the circulation sub, you can control the amount of fluid that is diverted.

The flow split at a circulation sub is determined as the fluid chooses the path of least resistance. The rates of flow through the circulation sub and down the string are determined when these two flow paths reach a pressure balanced state. When fluid inside pipe travels to the circulation sub, it faces 2 choices. The first one is to flow downward through the pipe and up the annulus. Let us call this Flow Path A. The alternative path is sideways through the circulation sub’s nozzles. We will call this Flow Path B.

As illustrated by Figure 3, Flow Path A involves a long, but wide conduit, while Flow Path B is an array of short constrictions (nozzles).

The circulating fluid does not have a preference as which path to flow. When the fluid passes the circulation sub, it senses the resistances of both paths and chooses the split of fluid so that it yields an overall minimum resistance.

The frictional pressure loss, or flow resistance, along Path A is a function of fluid viscosity, density, flow rate, pipe ID, hole ID, pipe OD and flow path length. On the other hand, the resistance of Path B is dominated by the pressure drop across the nozzles, which is reversely proportional to the square of the TFA of those nozzles. As we increase the TFA of a circulation sub, it becomes much easier for fluid to flow through Path B. As a result, less fluid will flow through Path A and the frictional pressure losses in the lower pipe and annular sections will be reduced. Whatever the percentage of flow split, the pump pressure and ECD of the system are both reduced by the fluid bypass.

In our example, we increase the TFA from 0, representing a case of no circulation sub, to 2 (in2). Figure 4 shows increased fluid bypass ratios as the TFA increases for 3 flow rates, 2 (bpm), 4 (bpm) and 6 (bpm). The circulating sub bypass ratio is the percentage of flow exiting the string through the circulating sub nozzles, as opposed to the bit.

Figure4: Circulation Sub Bypass Ratio vs TFA

Accompanying these increased bypass ratios, both the pump pressure and bottom hole ECD reduce rapidly at beginning and more gradually later, as shown in Figure 5 and 6, respectively. The pump pressure is reduced by almost 80% when TFA is increased from 0 (in2) to 1 (in2) for a flow rate of 6 (bpm). Meanwhile, for the same flow rate, bottom hole ECD is reduced by 7.6%. Further increase of TFA from 1 (in2) to 2 (in2) yields only marginal reduction.

Figure5: Pump Pressure vs TFA

Figure6: Bottom Hole ECD vs TFA

2. Effect of Circulation Sub Depth

The location of the circulation sub affects the overall downhole hydraulics. A circulation sub establishes a communication path between fluid inside the pipe and fluid in the annulus. The closer a circulation sub is to surface, the greater the fluid bypass ratio is, because Flow Path A is getting longer and creates a higher frictional pressure drop. Figure 7 shows the bypass ratios at various circulation sub locations along the wellbore. As expected, if we place the circulation sub at the bottom of the pipe, it would have no effect on pump pressure or bottom hole ECD.

Figure7: Circulation Sub Bypass Ratio vs Circulation Sub Depth

To take advantage of its unique characteristic for wellbore cleanup operations, a circulation sub is often placed at the depth where the wellbore geometry changes, such as the previous casing shoe. By increasing the pump rate, the hole section below the circulation sub with a smaller annular clearance can maintain the required fluid velocity from the downward split flow. The velocity of the fluid in the larger OD annulus above the circulation sub will see both the flow rate traveling down the string and through the sub’s ports, increasing the annular velocity to closely match that in the narrow clearance hole below.

Greater reductions in both the pump pressure requirement and bottom hole ECD are achieved when a circulation sub is placed closer to surface, as seen in Figures 8 and 9. The pressure and ECD drops because less fluid is traveling through the narrower clearance section of the annulus.

Figure8: Pump Pressure vs Circulation Sub Depth

Figure9: Bottom Hole ECD vs Circulation Sub Depth

# A New Drilling Engineering Toolbox App

Every drilling engineer and technician knows that performing drilling engineering calculations can be a tedious and time-consuming process, considering the many researches they have to do in order to find the right results. At times, even these results are not quite exact. While technology has tremendously driven the oil and gas industry forward with the creation and development of advanced software, most of the time these are meant to be used in the office and are not so simple and portable. To assist with this issue, Pegasus Vertex, Inc. has launched the new DR.DE Lite mobile app for both Android and Apple devices. This free app was developed to provide drilling engineers and technicians with a simple, useful, paperless and portable tool to make their drilling engineering calculations easier and faster.

The DR.DE Lite app is equipped with 29 functions that are divided into 11 groups.

From this great selection of functions, this tool can help you save time and efforts to get the calculations you need. In the past, drilling engineers and technicians had to obtain the information from different sources and after gathering the information manually perform the calculations and that could take up to an hour; however, now it can be done in a matter of seconds and the results are precise.

In this new tool, every function comes with three main buttons: Default, Clear and Calculate. The “Default” button gives users an example of standard input data. The “Clear” button allows users to delete all input data and the “Calculate” button performs the calculations based on the data users enter. The app collects all the information the users provide and in case the wrong data is entered, it will give a message as a warning.

For example, the “Hydraulic’ group contains four sub-functions: 1. ECD, which calculates the ECD at depth. 2. Hydrostatic pressure, which calculates the hydrostatic pressure of the mud column in a deviated well. 3. Pressure drop (Bingham) annulus, which calculates the pressure drop in an annular section for Bingham plastic fluids. 4. Pressure drop (Power law) annulus, which calculates the pressure drop in an annular section for power-law fluids.

In the “Pressure drop” function, users can specify the wellbore configuration, as well as fluid properties and flow rates. DR.DE Lite performs calculations for fluid velocity, flow pattern, pressure drop, Reynolds number and other key parameters for different flow patterns.

# Ingenuity Requires Persistence

There are several quotes from famous authors that talk about how “ingenuity can lead to significant rewards and how “persistence can be the key to survival. However, how high are the stakes when it comes to ingenuity and persistence in order to obtain significant rewards?

On the path towards any goal worth striving for, there are going to be obstacles, however, these obstacles cannot stop you from reaching your destination, it’s an opportunity to prove just how much this goal means to you. These are the moments in which to engage in persistence where you simply refuse to be deterred by the hurdles that one finds on any journey worth taking.

Increasing sophisticated and technological software is always the main goal; but simplicity and functionality are without a doubt very essential aspects of a software that must be kept in mind at all times during the developing process.

Technology is advancing unimaginably in today’s world. Each day new technology comes to live and brings the world better products. The drilling industry faces similar expectations and its growth isn’t just demanding but also fundamental for survival. With this in mind, we feel that our drilling software aims to achieve those goals fully. With the help of PVI software, companies worldwide can benefit not only by saving time, but also by saving money while making their operations more efficient and effective.

As an example of how ingenuity requires persistence let’s talk about HYDPRO, our drilling hydraulics model.

When drilling a vertical or an extended-reach well, if the equivalent circulating density (ECD) is either too low or too high, it can cause serious problems or compromise the project. For this reason a successful drilling operation largely relies on achieving the optimal ECD, and an optimized drilling hydraulics software can allow engineers to plan ahead, improve drilling efficiencies and reduce the risks. HYDPRO is a well-developed model, but when it comes to its user-friendly design as well as the interfaces, users will find it very simple and operate it effectively. It provides 3D visualization for planning, as well as summaries and analysis results that can be generated in Word, Excel, and PowerPoint formats in no time.

HYDPRO covers all aspects of hydraulics, for instance:

• Downhole circulating pressures
• Surge and swab
• Equivalent circulation densities (ECD)
• Bit optimization
• Hole cleaning
• Cuttings concentration
• Volumetric displacements

Thanks to a software like HYPDRO, the downhole drilling conditions can be cautiously examined and any potential problems can be identified prior to the job execution.

Today, our computer modeling is getting more advanced and the results are clearly worthwhile, because in the end is all about focusing on ingenuity and focusing on persistence.

# The Whole Package in a Small Package

The phrase “The Whole Package” is defined as:

1. All the elements constituting a whole or occurring as a unit.
2. Something or someone that possesses a full set of relevant characteristics.

This expression is often used to describe something that is of good choice; something that has everything that one needs for a specific matter or something that possesses all that one is searching for.

With all the advancements technology has achieved, it is marvelous how it keeps evolving in ways that are far beyond describable. Now you can have computer programs that can do multitasks; for instance, a software that possesses a full set of relevant characteristics for a specific need. It is literally getting the whole package in a small package.

Let’s talk about one good example of this.

DEPRO, a software developed by PVI, is a complete torque, drag and hydraulics program that combines all the essential components of our TADPRO and HYDPRO software.

For torque and drag, this software eliminates and diminishes a lot of the risks in drilling, completion or any particular tool operations. It also performs comprehensive hydraulics calculations so that the downhole drilling hydraulic conditions can be examined, and potential problems can be identified in advance before field execution.

Among its many features DEPRO also includes:

• Soft and stiff string models
• Buckling criteria
• Drilling, back reaming, rotating and tripping operations
• Minimum WOB to buckle
• Stress calculations
• 2D/3D animations
• Liner cementing job
• Field data comparisons on torque, drag, ECD and SPP
• Fixed flow rate analysis
• Surge and swab
• Hydraulics sensitivity analysis

This is a good package for service companies and operators in which they can find everything that they need for their jobs to be done all in one place. In other words they can have the whole package in a small package.

# 5 Benefits of Drilling Software Training

These days software, in general, becomes more and more user-friendly, even as it gets more and more sophisticated. We, as users, also have more confidence in using software without being bothered to read manuals or attend training sessions. Our busy schedules do not allow us to attend software training.

The situation is the same for drilling software users. As drilling engineers, whether we work in oil companies or in service companies, we always have next wells to drill, meetings to attend and deadlines to meet. We are so busy focusing on output that sometimes we forget about taking the time to receive input and recharge ourselves.

Unlike MS Office, drilling engineering software is not general-purpose software. It requires more specific knowledge to obtain results and understand the meanings of those results. Drilling software training should always be the first step to effectively use a software.  Here are the reasons why software training is necessary:

## 1. Understand the engineering models

All drilling engineering software is based on certain mathematical and engineering models. Understanding these models behind the graphic user interfaces helps us acknowledge the limitations of the software as well as the capabilities.

## 2. Interpret data meaningfully

Drilling software needs input data such as survey, BHA, wellbore and mud properties to predict hook load, pump pressure and ECD, etc. Like other software, only meaningful input data can lead to meaningful output data. It is necessary for users to know what the required input data is and to explain the meanings of the output data and charts to their colleagues or clients.

## 3. Increase efficiency

Training helps potential users to see the big picture as well as the tricks and features of the software. Once users know more aspects of the drilling software, they can use the software more efficiently. For example, survey data is essential to describe the well path and calculate the torque, drag and hydraulics. Survey data can be in various formats, Excel, text or PDF. All of our software can import data from these formats. The survey import feature saves tremendous time, especially when importing survey data from a PDF file.

## 4. Gain confidence

Software training connects the users with the developers of the software. The 2-way communication helps the users understand the ideas and structures of the software. Once users understand the inside calculations, the software is no longer a black box to them. Training removes the mysterious veil of the software. Users become more confident on explaining the results to clients.

## 5. Guide and influence

The software development training also provides a platform for users to influence and even guide the software development. Users, involved in field operations, can give suggestions, which can enhance the functions. This leads to better versions and win-win situations for software vendors and users.

Ancient Chinese philosopher Confucius once said:

“I hear and I forget.

I see and I remember.

I do and I understand.”

How true this is to drilling software training.

# Paper and E-reader vs. Book and HYDPRO

“Paper: The original wireless communication”. This is a sentence in Sappi Fine Paper North America’s eQ Journal 004. So interestingly stated, it tells a lot about human being’s communication need in the age of a digitalized world. “The World Wide Web, far from decreasing paper consumption, served to increase the amount of printing done at home and in the office.” Abigail J. Sellen and Richard H. R. Harper wrote in “The Myth of the Paperless Office”. After people began to use e-mail in the 1990s, paper consumption is estimated to have increased by some 40%. Neither is eBook device nor screen likely to make print on paper obsolete.

For my own reading pleasure, I use these 2 devices whenever I travel.

The convenience of carrying hundreds of books in one device allows me to switch between the books with a great flexibility, although I still enjoy reading paper books in my backyard. The tactile sensation of fingers turning fine-grain papers and the smell of ink and paper are also parts of reading experience. Developed 2000 years apart, paper and digital technologies not only co-exist, but also enhance the synergy between them as paper is routinely converted to digital documents and digital documents to paper.

We also see this 2-way conversion in the engineering efforts of drilling software developments. In 2012, Prof. Boyun Guo from University of Louisiana at Lafayette and I jointly wrote a book about drilling hydraulics (Applied Drilling Circulation Systems).

This book covers many areas of hydraulics concerns of drilling and mud engineers, with theories, examples, and operation guidelines: a good source of information and knowledge for petroleum engineering students and engineers.

Quite a few years ago, PVI developed its popular drilling hydraulics software HYDPRO which addresses majority of drilling hydraulics including frictional pressure drop, ECD, bit optimization and hole cleaning etc.

Similar technologies, delivered in traditional paper and the state-of-the-art application, serve drilling engineers’ essential need to understand and optimize drilling hydraulics. Equations in the book are the DNA of the HYDPRO, while the software is the screen play of the book.

Drilling software like HYDPRO is a great vehicle to carry the heavy loads of technologies. Yet, it provides engineers with easy-to learn and simple-to-use experience.