Page tree
Skip to end of metadata
Go to start of metadata


In linear formation approхimation the pressure response to the varying rates in the offset wells is subject to convolution equation:


p_n(t) = p_{0,n} + \sum_{k = 1}^N \sum_{\alpha = 1}^{N_k} \big( q^{(\alpha)}_k - q^{(\alpha-1)}_k \big) \ p^u_{nk}(t - t_{\alpha k})

where




1

p_n(t)

pressure at n-th well at arbitrary moment of time t

2

p_{0,n}

initial pressure at n-the well

3

q^{(\alpha)}_n

rate value of \alpha-th transient at n-th well

4

p^u_{nk} (t)

pressure transient response in n-th wel to unit-rate production from k-th well

5

t_{\alpha k}

starting point of the \alpha-th transient in k-th well

6

N

number of wells in the test
7

N_k

number of transients in k-th well

with assumption:

  • q^{(-1)}_k = 0 – for any well k = 1.. \ N

  • p^u_{nk}(\tau) = 0 at \tau < 0 for any pair of wells n, k = 1.. \ N


Hence, convolution is using initial formation pressure p_{i, n}, unit-rate transient responses of  wells and cross-well intervals p^u_{nk} (t) and rate histories  \{ q_k (t) \}_{k = 1 .. N} to calculate pressure bottom-hole pressure response as function time p_n(t):

(1) \big\{ p_{0, n}, \{ p^u_{nk} (t), q_k (t) \}_{k = 1 .. N} \big\} \rightarrow p_n(t)


The RDCV is a reverse problem to convolution and search for N^2 functions p^u_{nk} (t)  and N numbers p_{i, n}  using the historical pressure and rate records \{ p_k(t), \ \{ q^{(\alpha)}_k \}_{\alpha = 1.. N_k} \}_{k = 1 .. N} and provides the adjustment to the rate histories for the small mistakes  \{ q_k \}_{\alpha = 1.. N_k} \rightarrow \{ \tilde q_k \}_{\alpha = 1.. N_k}:

(2) \big\{ p_k(t), q_k (t) \big\} _{k = 1 .. N} \rightarrow \big\{ p_{0, n}, \{ p^u_{nk} (t), \tilde q_k (t) \}_{k = 1 .. N} \big\}

The solution of deconvolution problem is based on the minimization of the objective function:

(3) E(\{ p_{0,n}, p^u_{nk}(\tau), q^{(\alpha)}_n \}_{n=1..N}) \rightarrow {\rm min}

where

(4) E(\{ p_{0,n}, p^u_{nk}(\tau), q^{(\alpha)}_n \}_{n=1..N}) = \sum_{n=1}^N \Big(p_{0,n} + \sum_{k = 1}^N \sum_{\alpha = 1}^{N_k} (q^{(\alpha)}_k - q^{(\alpha-1)}_k ) \ p^u_{nk}(t - t_{\alpha k})- p_n(t) \Big)^2 + w_c \, \sum_{n = 1}^N \sum_{k = 1}^{N_k} {\rm Curv} \big( p^u_{nk}(\tau) \big) + w_q \, \sum_{k = 1}^N \sum_{\alpha = 1}^{N_k} \big( q^{(\alpha)}_k - \tilde q^{(\alpha)}_k \big)^2

and objective function components have the following meaning:



\sum_{n=1}^N \Big(p_{0,n} + \sum_{k = 1}^N \sum_{\alpha = 1}^{N_k} (q^{(\alpha)}_k - q^{(\alpha-1)}_k ) \ p^u_{nk}(t - t_{\alpha k})- p_n(t) \Big)^2

is responsible for minimizing discrepancy between model and historical pressure data

w_c \, \sum_{n = 1}^N \sum_{k = 1}^{N_k} {\rm Curv} \big( p^u_{nk}(\tau) \big)

is responsible for minimizing the curvature of the transient response (which reflects the diffusion character of the pressure response to well flow)

w_q \, \sum_{k = 1}^N \sum_{\alpha = 1}^{N_k} \big( q^{(\alpha)}_k - \tilde q^{(\alpha)}_k \big)^2

is responsible for minimizing discrepancy between model and historical rate data (since historical rate records are not accurate at the time scale of pressure sampling)


In practice the above approach is not stable.

One of the efficient regularizations has been suggested by Shroeter 


One of the most efficient method in minimizing the above objective function is the hybrid of genetic and quasinewton algorithms  in parallel on multicore workstation.

The RDCV also adjusts the rate histories for each well \{ q^{(\alpha)}_k \}_{\alpha = 1.. N_k} \rightarrow \{ \tilde q^{(\alpha)}_k \}_{\alpha = 1.. N_k} to achieve the best macth of the bottom hole pressure readings.


The weight coefficients w_c and  w_q  control contributions from corresponding components and should be calibrated to the reference transients (manuualy or automatically).


The RDCV methodology constitute a big area of practical knowledge and not all the tricks and solutions are currenlty automated and require a practical skill. 


See also


Petroleum Industry / Upstream / Subsurface E&P Disciplines / Production Analysis (PA) / Pressure Deconvolution / Multiwell deconvolution (MDCV) / RDCV




  • No labels