The general form of non-linear single-phase pressure diffusion @model with the finite number of sources/sinks is given by:

(1)	$\begin{array}{l}\displaystyle \phi \cdot c_t \cdot \partial_t p + \nabla {\bf u} + c \cdot ( {\bf u} \, \nabla p) = \sum_k q_k(t) \cdot \delta({\bf r}-{\bf r}_k)\end{array}$

(2)	$\begin{array}{l}\displaystyle {\bf u} = - M \cdot ( \nabla p - \rho \, {\bf g})\end{array}$

where

$\begin{array}{l}p(t, {\bf r})\end{array}$	reservoir pressure	$\begin{array}{l}t\end{array}$	time
$\begin{array}{l}\rho({\bf r},p)\end{array}$	fluid density	$\begin{array}{l}{\bf r }\end{array}$	position vector
$\begin{array}{l}\phi({\bf r}, p)\end{array}$	effective porosity	$\begin{array}{l}{\bf r }_k\end{array}$	position vector of the $\begin{array}{l}k\end{array}$ -th source
$\begin{array}{l}c_t({\bf r},p)\end{array}$	total compressibility	$\begin{array}{l}\delta ( \bf r )\end{array}$	Dirac delta function
$\begin{array}{l}q_k(t)\end{array}$	sandface flowrates of the $\begin{array}{l}k\end{array}$ -th source	$\begin{array}{l}\nabla\end{array}$	gradient operator
$\begin{array}{l}M = k / \mu\end{array}$	phase mobility	$\begin{array}{l}{ \bf g }\end{array}$	gravity vector
$\begin{array}{l}k\end{array}$	formation permeability to a given fluid	$\begin{array}{l}{ \bf u }\end{array}$	fluid velocity under Darcy flow
$\begin{array}{l}\mu\end{array}$	dynamic viscosity of a given fluid

Derivation

The alternative form is:

(3)	$\begin{array}{l}\displaystyle \phi \cdot c_t \cdot \mu \cdot \partial_t \Psi - \nabla \cdot \left( k \cdot \Big( \vec \nabla \Psi - \frac{\rho^2}{\mu} \, {\bf g} \Big) \right) = \sum_k q({\bf r}) \cdot \delta({\bf r}-{\bf r}_k)\end{array}$

where

$\begin{array}{l}\displaystyle \Psi(p) =2 \, \int_0^p \frac{p \, dp}{\mu(p) \, Z(p)}\end{array}$	Pseudo-Pressure
$\begin{array}{l}\mu(p)\end{array}$	dynamic fluid viscosity
$\begin{array}{l}Z(p)\end{array}$	fluid compressibility factor

Derivation

Physical models of pressure diffusion can be split into two categories: Newtonian and Rheological (non-Newtonian) based on the fluid stress model.

Mathematical models of pressure diffusion can be split into three categories: Linear, Pseudo-linear and Non-linear.

These models are built using Numerical, Analytical or Hybrid pressure diffusion solvers.

Many popular 1DR solutions can be approximated by Radial Flow Pressure Diffusion @model which has a big methodological value.

The simplest analytical solutions for pressure diffusion are given by 1DL Linear-Drive Solution (LDS) and 1DR Line Source Solution (LSS)

The table below shows a list of popular well and reservoir pressure diffusion models.

Wellbore storage model	Well model	Reservoir model	Boundary model
Constant	Skin-factor	Homogeneous	Infinite
Fair	Vertical well	Dual-porosity	Circle No Flow
Rate-dependant	Fractured vertical well	Dual-permeability	Circle Constant P_i
	Limited entry well	Anisotropic reservoir	Single fault
	Horizontal well	Multi-layer reservoir	Parallel faults
	Slanted well	Linear-composite	Intersecting Faults
	Multifrac horizontal well	Radial-composite

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See also