What exactly is transient and how is different from steady-state calculations?

The important effect of Temperature.

The transient Temperature effect is very important in the accuracy of MPD/UBD calculations. Most people ignore this effect perhaps because they don’t have the right tools to calculate the transient effect, so many engineers just take the thermal gradient and use steady state formulas to calculate the bottom hole temperature.

What exactly is transient & how is it different from the steady-state vector calculator?

Transient phenomena are always presented during oil-well drilling, as an implicit result of changing the flow rate, pumping, and choke pressures while the fluid “mud” is circulating through the well. The process of circulation consists of 3 stages: (1) fluid enters the drill pipe with a known flow rate and specified temperature and flows down, (2) fluid exits the drill pipe through the bit and enters the annulus at the bottom, (3) the fluid flows up in the annulus and exits the annulus. There is a cross flow and therefore heat transfer between the fluids in the drill pipe and annulus via convection and also conduction through drill pipe. The annulus fluid temperature is determined by the rate of heat convection up the annulus, the rate of heat exchange between the annulus and drill pipe wall, and the rate of heat exchange between the formation and annulus fluid. The friction across the drilling system caused by hydraulic pressure loss also is one of the heat sources which are transferred by the force. Generally, the formation temperature is lower than the annulus fluid temperature at locations closer to the surfaces; however, for deeper locations formation temperature exceeds the annular fluid temperature. Therefore, circulation of mud cools down the near wellbore formation for deeper locations. Factor affects transient calculations:

  • Duration of the drilling or circulation process
  • Well geometry
  • Thermophysical and transport properties of the drilling mud and formation
  • Heat exchange between all zones

Steady states vector calculator have many assumptions:

  • Heat transfer between annular fluid and formation is linear or constant.
  • Neglect vertical heat transfer
  • Formation is homogeneous
  • Neglect thermophysical fluid’ properties.

Onshore Temperature Profile

steady state vector calculator   Let’s take a closer look at the Onshore wells (Graph 1) Blue arrows. You can notice during the circulation the lower annular zone cools down. The cooling effect depends on the flow rate, the physical properties of the fluid particularly mud density, specific heat and thermal conductivity. Also the heat exchange between the zones. Some fluids can return cooler or warmer depending if the well is deeper or shallow.

Offshore Temperature Profile

steady state vector calculator On the other hand the Offshore well the temperature profile changes considerable. First the negative temperature gradient in the ocean makes the temperature get event cooler then at the seabed the temperature changes because of the positive temperature gradient. When starts circulation the lower annular zone get cooler, however at some point in the annular it’s hotter than formation temperature. In the Riser is getting cooler again and might return cooler or hotter that surface temperature depending on the well geometry (deeper/shallow) and physical fluid’s properties among other conditions.

Why Temperature is significant?

Well Control:

Let’s consider a gas influx (Kick); generally the “PV= Constant” assumption appears to be more or less acceptable in evaluating a shallow gas kick. The perfect gas law “PV=nRT” improves the predictions including the effect of temperature. Here is when the Temperature plays an important role in our kick calculations.

As you can see the gas density is not constant during the influx control. Temperature changes circulating out the influx.

Thermal Expansion:

Thermal expansion of drilling muds, in areas with high geothermal gradients, can lead to unintentional underbalance-a kick may occur. In deep, hot wells, the densities of water and oil-based muds can vary significantly from the measured values at surface conditions. Calculations have shown that bottom hole pressures, predicted with constant surface density, are often in error by hundreds of psi.

Hydrates formation:

Another effect of the temperature is the formation of hydrates; studies have shown the hydrate formation during well-control and drilling operations are increasing. Below graph shows the hydrate formation region in offshore well. So modeling the transient temperature in offshore wells is very important to be aware if it’s possible avoid hydrates formation.

Now you know the importance of modeling a well using transient simulations. Drillsoft has developed a Transient Hydraulic Model you can easily plan your well successfully with a few clicks in a very ease-to use UI. Here a video showing the Transient Temperature Modeling in an Offshore well. https://youtu.be/gIHi4BCyoPU

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