Looking for The Right Key

“Many people are confused with complexity and shy off from it. Strangely, complexity arises from simple rules, but people are finding it difficult to grasp. However, you need to move out of this situation soon before it gets out of control. Do what ants do: they diverge to look for new sources of food and then converge once a new path has been identified. Look for new ideas till you identify a new path or a new pattern to help you find the right key for the right lock that opens up the right door.”

        - Ali Anami

As the 21st century passes, the oil industry continues to look for new ways to innovate and increase its productions. It continues to search for reserves in more complex environments, especially in deep water. Already many fields are slated for development in both shallow and deep offshore wells and new solutions and technology are needed to meet these goals.

As a reservoir depletes, the pressure and production rates decline and lost circulation or differential sticking problems can prevent the drilling operations from increasing their production. Underbalanced drilling (UBD) is a practical method to drill in such depleted or low-pressure areas. One of the many benefits of underbalanced drilling is that it can prevent or reduce near wellbore permeability damage; as a result, it enhances the production, delaying water, gas or oil leaks due to the lower pressure drop that is required for the same flow rates.

The advantages of Underbalanced Drilling are:
  • Improves drilling performance
  • Increases penetration rate (ROP)
  • Extends bit life
  • Minimizes differential sticking of pipe
  • Diminishes lost circulation
  • Reduces formation damage
  • Simplifies early production while drilling
  • Allows formation evaluation and tests while drilling
  • Higher productivity completions
  • Picks total depth (TD) from inflow performance
  • Closed, pressure-controlled system

In order to design and achieve a successful underbalanced drilling project many important elements must be taken in consideration and there are 4 steps that have to be applied to determine the options and requirements for UBD: This is particularly important for underbalanced drilling operations offshore from floating vessels, where critical issues can increase significantly.

  1. Determine Bottom Hole Pressure (BHP) requirements.
  2. Identify the drilling fluid options.
  3. Establish the well design and perform flowing modeling.
  4. Select the surface equipment.

An advance software model has been developed which incorporates these elements, including multiphase flow calculations, gas and liquid injection rate optimization, hydrostatic gradient and frictional pressure loss calculations, cuttings transport, reservoir fluid influx (oil, gas, or water), and operational procedures such as tripping and liquid unloading.

Just like the rest of PVI's software, UBDPRO is the right tool when it comes to increasing a well’s productivity. It’s all about looking in the right direction (for the right key) and making the efforts to obtain it.

UBDPRO - Underbalanced Drilling Hydraulics

Floating Egg

I did a simple science project with my 8-year-old daughter to show the action of buoyancy.

Step 1: Put a raw egg in a glass full of water. The egg sinks to the bottom.

Step 2: Keep adding salt and stir the water.  You will feel guilty of wasting too much salt, but eventually, the egg arises.

Before / After Adding Salt

My daughter and I were both amazed by this simple demonstration of buoyancy of salt water. Recall that Archimedes’ principle states that the upward force on a submerged object (egg) is equal to the weight of the water that it displaces. Salt water weights more than fresh water, this difference of upward force (buoyancy) makes the egg arise.

In drilling mud situations, saltwater drilling fluids can be formulated with high-density brines, such as calcium chloride, calcium bromide, etc. The mud weight is dependent upon the specific gravity (SG) of salts added and concentration of these salts. The heavier the mud, the higher the bottom hole pressure is. Note that one of the main functions of drilling fluids is to provide hydrostatic pressure to prevent formation fluids from entering into the well. Heavy mud brings some advantages and some disadvantages to other aspects of drilling operations because of the buoyancy it creates.

Buoyancy makes drill string appear light. This reduces the side force, which pushes drill string against wellbore. Therefore, there is less torque and drag for a heavier drilling fluid system, but what helps also causes problems in this case: too much buoyancy revokes the effectiveness of transferring the pipe weight to the bit.

When cementing a casing, due to the large size of the casing and the big difference of fluid densities in the annulus and inside the pipe, the buoyancy could be so great that the casing could be pushed out of the hole if it is not chained down.

Let’s use Dr.DE (drilling engineering tool box software) to show a couple of examples.

Consider an 11.75” OD casing with weight of 47lb/ft. The casing shoe is at 12,345 ft in a vertical well. The annulus is full of cement slurry of 16ppg. We assume mud weight of 8.33 ppg. With the help of Dr.DE the calculation shows that the casing will be lifted up.

Casing will be lifited up | Dr.DE - drilling toolbox

Casing Will Be Lifited Up

Now, change the mud weight to 9 ppg, due to this change, the hydraulic pressure inside the casing increases and the casing will not be lifted up by the buoyancy.

Casing will NOT be lifited up | Dr. DE Drilling Toolbox

Casing will NOT be lifited up