# Evaluating fracture volume loss during production process by comparative analysis of initial and second flowback data

## TL;DR

This paper compares initial and second flowback data to evaluate how fracture volume changes during production and preloading processes in oil wells.

## Contribution

A novel comparative analysis of flowback data to estimate fracture volume loss and drive mechanisms during production.

## Key findings

- Effective fracture volume during second flowback is 3%–45% lower than during initial flowback.
- The compaction-drive index decreases by 16% during second flowback, compensated by higher hydrocarbon-drive index.
- Fracture pressure depletion during production can last up to 800 days, causing volume loss.

## Abstract

The fracture volume is gradually changed with the depletion of fracture pressure during the production process. However, there are few flowback models available so far that can estimate the fracture volume loss using pressure transient and rate transient data. The initial flowback involves producing back the fracturing fluid after hydraulic fracturing, while the second flowback involves producing back the preloading fluid injected into the parent wells before fracturing of child wells. The main objective of this research is to compare the initial and second flowback data to capture the changes in fracture volume after production and preload processes. Such a comparison is useful for evaluating well performance and optimizing fracturing operations.

We construct rate-normalized pressure (RNP) versus material balance time (MBT) diagnostic plots using both initial and second flowback data (FBi and FBs, respectively) of six multi-fractured horizontal wells completed in Niobrara and Codell formations in DJ Basin. In general, the slope of RNP plot during the FBs period is higher than that during the FBi period, indicating a potential loss of fracture volume from the FBi to the FBs period. We estimate the changes in effective fracture volume (Vef) by analyzing the changes in the RNP slope and total compressibility between these two flowback periods. Vef during FBs is in general 3%–45% lower than that during FBi. We also compare the drive mechanisms for the two flowback periods by calculating the compaction-drive index (CDI), hydrocarbon-drive index (HDI), and water-drive index (WDI). The dominant drive mechanism during both flowback periods is CDI, but its contribution is reduced by 16% in the FBs period. This drop is generally compensated by a relatively higher HDI during this period. The loss of effective fracture volume might be attributed to the pressure depletion in fractures, which occurs during the production period and can extend 800 days.