**1. Introduction**

As regulatory restriction on the disposal of solid wastes such as drilling cuttings becomes increasingly tightened worldwide, cuttings injection (CI) into an existing well or a dedicated well becomes standard and required operation in drilling. Various guidelines and best practices in CI operations were established to accommodate local conditions and requirements. A systematically designed CI operation can be seen in reference [1].

**2. Quality of pressure maintenance in CI operations**

**•** Period A – gradual pressure build up and difficulty in fracture closing

**•** Period D - gradual pressure build up and difficulty in fracture closing

responses can be divided into:

pressure decline curves.

**•** Period B – good fracture closure

**•** Period C - excellent fracture closing

without resourcing to its physical origins.

pressures are seen in injection periods B and E.

build up due to insufficient fracture closing.

Figure 1 provides a long-term assessment on batch cuttings injection. Four periods of pressure

Importance of Fracture Closure to Cuttings Injection Efficiency

http://dx.doi.org/10.5772/56070

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Periods A and D form a repeat cycle with slightly higher peak pressures in Period D than in Period A. It is easily seen that the longest time for pressure dissipation was during Period C. This also led to the best possible fracture closure and lowest terminal pressure among all

The physics of fracture closing and its relation to pressure response can be more complex than from the stated single event or relation alone. For example, the fracture could be closed on cuttings so that the terminal pressure is high. In general, a slow leakoff can be envisioned as the equivalence of difficult in fracture closing. The sufficient pressure dissipation can also be caused by the initiation of a new fracture in a different orientation from the previous one, or an increase in fracture aperture due to its connection to natural fractures, or anything else. For simplicity in this paper, however, we contribute the pressure dissipation to the fracture closure

Figure 2 shows the pressure responses from 28 batches of cuttings injection separated into five periods (A-E). Significant rising peak pressures during Periods A, B, C and E reflect difficulties in cuttings injection. This is in contrast to the smaller peak pressure increase in Period D that indicates relatively easy injections. A couple of good pressure relief or deep drop terminal

**Figure 1.** Pressure responses over long period of cuttings injection. (A): gradual pressure build up due to insufficient fracture closing; (B): good fracture closing; (C): excellent fracture closing with sufficient time; and (D): gradual pressure

One of the very important tasks to ensure a successful CI operation is to continuously monitor injection pressure behavior. Among various pressure responses, the characteristics of fracture closure after shut-in directly relate to the quality of pressure control and thus to the ability to prevent near-well screen outs and unexpected shut down of injection wells.

Fracture closure can be assessed empirically by capturing the inflection point in the pressure decline curve. The inflection point generally reflects the transition between linear or bi-linear flow within the hydraulic fracture and pseudo-radial flow outside the hydraulic fracture.

Fracture closure can also be evaluated more accurately using analytical methods. Among various methods, popular ones are: a) the square-root time method to determine the transition between bi-linear flow and pseudo-radial flow using the filtration theory proposed by reference [2]; b) the Horner time method to determine early time fracture flow and late time pseudo-radial flow (see reference [3]); c) the *G* function method to identify the onset of fracture closure by examining the transition of the flow pattern proposed by Knolte (see reference [4]); and d) the superposition *G* function derivative method to determine the reversal of the *G* derivative that signals the transition between bi-linear flow and pseudo-radial flow proposed by Barree (see reference [5]).

This paper is intended to explore the physics of fracture closure behind the pressure decline curves. By examining the pressure responses in the CI pressure monitoring cases, patterns of both successful and unsuccessful pressure control were captured. Observations reveal that the length of duration in the shut-in period between the injections is a critical parame‐ ter with respect to the quality of pressure dissipation and fracture closure. Cuttings injection efficiency is a function of the magnitude of the disposal domain of the stimulated fracture volume.

Sensibly interpreting the physics of fracture closure from bottom-hole pressure responses can be difficult due to the inability to directly measure hydraulic fracture evolution. By comparing various fracture decline curves with reference to their relation to the *G* function and superpo‐ sition derivatives, this paper identifies the key parameter as the pressure decline curve shape. The conventional interpretation of pressure decline with regard to the physical behavior of hydraulic fractures appears to be insightful. However, verification of this interpretation continues to be a challenge to the current technology.
