What is ECD?: A Primer on What Affects Equivalent Circulating Density

I am cross-posting this between between my personal blog, Oilfield, Tech, and Stuff and the LinkedIn Pulse article system.

It appears to me that most people involved with drilling wells have a basic understanding of what Equivalent Circulating Density or ECD is. While having a basic understanding, most do not have a realize the different factors that affect ECD. So, I will detail below the main categories that effect ECD and provide a high-level explanation of how they achieve this.

I am also including a Three Letter Acronym (TLA) Dictionary for the terms used in this article due to sheer abundance of TLAs used in the Oilfield and so the casual reader can understand what they stand for.

TLA Dictionary:

TLA	Full Name	Category	Sub-Category 1	Sub-Category 2	Sub-Category 3	Sub-Category 4
ECD	Equivalent Circulating Density	Drilling	Fluids	Cementing	Tubulars	Geomechanics
ESD	Equivalent Static Density	Drilling	Fluids	Cementing	Tubulars	Geomechanics
IEF	Invert Emulsion Fluid	Drilling	Fluids	Geomechanics	Cementing
PV	Plastic Viscosity	Drilling	Fluids	Cementing
YP	Yield Point	Drilling	Fluids	Cementing
TVD	True Vertical Depth	Drilling	Fluids	Cementing	Tubulars	Geomechanics

The main categories that influence ECD are: 

• Flow Geometry
• Fluid Resistance to Flow
• Pressure of Flow
• Fluid Density
• Fluid Temperature 
• Acquired Solids

Flow Geometry - Bore hole geometry introduces pressure loss based on drag on the fluid as it passes through the various components of the fluid flow path (Standpipe, Drillstring Components, Open Hole, Casing, etc.) and hydrostatic pressure based on the True Vertical Depth (TVD)  of the point in question.

Fluid Resistance to Flow -  Resistance to flow is related to the rheology of the fluid. While the Plastic Viscosity (or PV) and Yield Point (or YP) play a part in the rheology, they are only a general measurement and are more accurate for quantifying a Newtonian fluid​, such as water, than a non-Newtonian fluid such as Invert Emulsion Fluids or IEF. On the other hand, the Herschel-Bulkley n, k, and Tau Zero calculations better represent an IEF for rheological purposes.

Pressure of Flow - The pressure at which a fluid is being forced to flow (pump pressure) has a big impact on ECD. The higher the pump pressure, the higher the ECD in most cases.

Fluid Density - Fluid density, by definition, is a main component of ECD and ESD.

Fluid Temperature - Fluid Temperature affects ECD due to thermal expansion in IEF. When the fluid heats up, the base fluid, usually some form of biological, natural, or synthetic oil, expands, thus resulting in more volume of base fluid per given amount of weighting material, resulting in a lower density overall for the fluid. BUT, when taken into account with the TVD portion of the geometry aspect, hydrostatic pressure also compresses the same base fluid, so both parameters need to be taken into account when calculating ECD.

Acquired Solids - Acquired solids are the final component discussed today. From a drilling perspective, this is drilled cuttings and or formation cavings that have been liberated from the formation and  carried in the fluid column.

I hope this helps to promote understanding of what factors influence ECD. Please  respond in the comments to let me know if this information is helpful. 

EDITED: The factors covered above are the basic influences and assume a vertical hole and does not address Managed Pressure Drilling (MPD) or the effect of drillstring rotation (RPM). MPD involves applying back pressure to the wellbore via the choke to maintain hydrostatic pressure on the wellbore. RPM has a minimal effect compared to the main factors detailed above. Drillstring Eccentricity (the drillstring's displacement from the center of the wellbore towards the wall of the wellbore, mostly seen in deviated/horizontal hole.)