Areal average reservoir pressure around a given location.


The averaging procedure is defined in several ways depending on applications and summarised in the table below:


Formation Pressure Defintion

Application

Shut-in formation pressure estimate based on a wellbore sandface pressure after a given well is shut-in for hours

p_e = p_{wf}(t_e) \bigg|_{q = 0}


Well intervention

This definition is based on the practical observation of wellbore pressure in shut-in wells for well intervention purposes.

It is the simplest and the most popular definition of formation pressure and is widely used in all upstream industry applications.

The definition of shut-in time is specific to each field or sometimes field area depending on the rock properties and the past well intervention experience.

Some conventions are to pick at the end of the radial flow, as the most common number from the past pressure tests, which makes this definition close (but still not equal) to the

Boundary-average formation pressure estimate along the boundary of drainage area

p_e = \frac{1}{L_e} \int_0^{L_e} p(x,y,z) dl

where is the boundary of drainage area

Pressure Testing

This definition is based on the idea that there is a boundary line which restricts radial flow around a well, which is a fair assumption in most practical cases.

The advantage of this method over is that:

  • it provides more accurate estimate of the pressure away from a given well
  • is not dependent on convention, which may be not valid for a given well


Field-average formation pressure estimate within the drainage area

p_r = \frac{1}{A_e} \iint_{A_e} p(x,y,z) dS


Well Flow Performance Analysis

This definition is based on the productivity index estimate and assumption that it stays constant

Historically this definition is marked with a different symbol instead of

9-cell formation pressure estimate from reservoir flow simulation model

p_{e9, \ i,j} = \frac{1}{9} \sum_{k=i-1}^{i+1} \sum_{l=j-1}^{j+1} p_{k,l}



Dynamic Modelling

It defines formation pressure as an average of numerically calculated pressure values in 9 grid cells of reservoir flow simulation model

It provides a very rough and often inaccurate estimate of formation pressure for planning new wells, workovers, well performance analysis and testing existing dynamic model against other estimates.

It is widely used in history matching as input parameter to match with, which maybe highly controversal providing the difference in.