Introduction

Rotational drilling with constant operating parameters is widely used at the moment in prospecting for mineral deposits. In the meantime, further improvement of this method requires new design and technology solutions. Transition to technologies using pulsed variation of drilling process parameters is one of such solutions.

Particularly, the developers of rotational drilling technologies manifest a keen interest to pulsed well flushing methods recently. For example, a mud-pulse rotational technology for drilling deep wells with abnormal formation pressure has been developed in the US (Kollé and Marvin 1999). An ability to increase penetration speed by use of intermittent jets for well flushing has been studied (Wang 2005; Liu et al. 2015). Results of experimental studies mentioned and numeric modeling allow assuming that rock destruction is performed more efficiently due to its weakening by jet mechanical impact and bottom-hole cavitation. Hydraulic pulse generators used to form frilling fluid supply mode has been considered (Qu et al. 2016; Shi et al. 2015).

A positive experience of application of pulsed flushing during diamond and hard-alloy drilling bits has been obtained in (Tungusov 2009). Authors have shown that using pulsed flushing promotes for better rock destruction and improves well cuttings removal efficiency (Filimonenko 2007; Gorshkov and Gorelikov 1992).

It has been shown by Fassahov et al. (Fassahov et al. 2005) that pulsed flushing allows saving power and resources in oil field operation. Therefore, using alternating drilling fluid supply allows boosting rock destruction as well as improving other drilling process conditions.

Taking into account that diamond drilling is one of the most common ways of prospecting for solid commercial deposits ((Wang et al. 2015).), research held for its improvement is considered a very important task.

General characteristic of pulsed flushing modes

Well drilling using pulsed flushing belongs to pulsed drilling technologies. Concepts, definitions, and classification of pulsed drilling technologies were proposed first by A. Kozhevnykov in investigation report of National Mining University (former Dnipropetrovsk Mining Institute) according to the agreement with National Institute for Exploration Methods and Technology performed. These results are presented in the form of a report at “Drilled rock mechanics” international conference (Kozhevnykov 1992). This issue is detailed by Kozhevnykov and Filimonenko (Kozhevnykov and Filimonenko 2010), the brief characteristics of pulsed drilling technologies are summarized as a chart in Table 1.

Table 1 Classification of pulsed drilling technologies
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According to chart in Table 1, there are three variants of pulsed flush drilling technology:

  • one single-parameter, with only one variable parameter Q;

  • two double-parameter, with two variable drilling parameters taken in pairs, Q and n or Q and F;

  • one triple-parameter (when all three drilling parameters are variable at a time, Q, n, and F)

In this way, pulsed flushing can be carried out independently as well as with other transient parameters. Consideration of pulsed well flushing leads to four possible ways of its implementation (see Fig. 1).

Fig. 1
figure 1

Pulsed flushing methods: with variable fluid flow (а); with pulsed fluid flow (b); with intermittent fluid flow (c); with revertive fluid flow (d)

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The following factors have been introduced to characterize pulsed well flushing.

Pulse factor kpulse, the ratio of fluid feed flow variation ΔQ to max fluid feed flow Qmax:

$$ {k}_{pulse}=\frac{\varDelta Q}{Q_{\mathrm{max}}} $$
(1)

Steadiness factor kst, the ratio that characterizes min fluid feed rate Qmin to max flow rate:

$$ {k}_{st}=\frac{Q_{\mathrm{min}}}{Q_{\mathrm{max}}}. $$
(2)

Pulse and steadiness factors are mutually-dependent, because

$$ {k}_{pulse}=\frac{\varDelta Q}{Q_{\mathrm{max}}}=\frac{Q_{\mathrm{max}}-{Q}_{\mathrm{min}}}{Q_{\mathrm{max}}}=1-\frac{Q_{\mathrm{min}}}{Q_{\mathrm{max}}}=1-{k}_{st}. $$
(3)

Hence,

$$ {k}_{pulse}+{k}_{st}=1. $$

Steadiness factor is also functionally constrained with fluid flow rate, because

$$ {k}_{st}=\frac{Q_{\mathrm{min}}}{Q_{\mathrm{max}}}=\frac{Q_{\mathrm{max}}-\varDelta Q}{Q_{\mathrm{max}}}=1-\frac{\varDelta Q}{Q_{\mathrm{max}}}=1-{k}_{pulse}. $$
(4)

Intermittency factor kint shows ratio of fluid feed pause duration tpause to fluid feed duration tflow

$$ {k}_{int}=\frac{t_{pause}}{t_{flow}} $$
(5)

Reversibility time factor ktrev shows ratio of times of fluid direct tdir and inverse feed tinv

$$ {k}_{rev}^t=\frac{t_{dir}}{t_{inv}} $$
(6)

Reversibility flow factor kQrev shows ratio of fluid direct (tdir) and indirect feed flow values (tinv).

$$ {k}_{rev}^Q=\frac{Q_{dir}}{Q_{inv}} $$
(7)

Bench testing of diamond core drilling with pulse flushing

One of pulsed well flushing methods, namely, intermittent flushing, has been held in bench-scale conditions in Drilling laboratory at Department of Technic Prospecting Of Deposits in National Mining University (Dnipro, Ukraine) (Kozhevnykov et al. 1990).

Drilling has been performed with a single-layer diamond core type 01А3, 59 mm dia, shown in Fig. 2, with four flushing channels.

Fig. 2
figure 2

Diamond drilling bit type 01А3; а general view; b top view; c side view

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Drilling was performed on a granite block with IX drillability category (Fig. 3). Water was used as drilling fluid.

Fig. 3
figure 3

Granite block used in a bench test drilling

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Rig type ЗИФ-300 (ZIF-300) equipped with auxiliary gear box together with drilling pump type НБ-3 (NB-3) are the test bench main components. ЭМР-2 (EMR-2) flowmeter was used additionally for recording and control of drilling conditions (Fig. 4).

Fig. 4
figure 4

Test bench general view. 1, rock drilling machine type ЗИФ-300 (ZIF-300); 2, kelly stem; 3, drilling tool; 4, computer; 5, power supply unit; 6, electronic interface; 7, granite block

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Test results of diamond drilling using intermittent flushing mode

Flushing mode was being changed during drilling with 01A3 core bit. Drilling was held using continuous flushing, respective intermittency factor kint = 0, as well as intermittent flushing with intermittency factor values of kint = 0.2 and kint = 1. Drilling was held with different rotation speed and axial load values. Results of experimental investigation of intermittent flushing impact on mechanical penetration speed are shown in Table 2.

Table 2 Results of test banch diamond drilling using core type 01A3 with intermittent flushing
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As it can be seen from Table 1, all examined intermittently flushed drilling modes provide penetration speed increase as compared to continuous flushing mode. Drilling efficiency is affected not only by flushing mode features, but also by numeric values of drilling parameters that provide resultant power on bottom-hole (Karakus and Perez 2014). There was no significant wear on the drill bit due to the relatively shorter drilling depth and the high performance of natural diamond in the bit.

Table 3 shows power values on bottom-hole that correspond to numeric values of rotational speed and axial load in this investigation.

Table 3 Test bench values of power on bottom-hole
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Efficiency of drilling using intermittent flushing as compared to continuously flushed drilling can be evaluated with an efficiency criterion Keff that presents a ratio of penetration rate using intermittent flushing Vint to penetration speed using continuous flushing Vconst.

$$ {K}_{eff}=\frac{V_{\mathrm{int}}}{V_{const}} $$
(8)

Penetration speed is connected with power on bottom-hole P by the relation.

$$ V=\frac{P}{A\cdot S}. $$
(9)

where A is specific energy consumption on rocks destruction process, S area of working face of borehole.

Let us write (1) by taking in account (2), in the form of

$$ {K}_{eff}=\frac{A_{const}}{A_{\mathrm{int}}}\frac{P_{int}}{P_{const}} $$
(10)

where subscript index “int” means the parameters taken at intermittent flushing mode, and index “const” means the parameters taken at constantly flushing mode. Let us introduce the coefficients which show ratio of downhole power and energy consumption by using intermittent and constant flushing mode

$$ {k}_P=\frac{P}{P_{const}} $$
(11)
$$ {k}_A=\frac{A_{\mathrm{int}}}{A_{const}} $$
(12)

Then the criterion of drilling efficiency with intermittent flushing depends on the ratio of these factors

$$ {K}_{eff}=\frac{k_N}{k_A}. $$
(13)

According to data provided in Table 2, efficiency factor varies from 1.18 to 2.2. Intermittent flushing with intermittency factor kint = 1 affects drilling more than with intermittency factor kint = 0.2. At the same time, power on bottom-hole value affects penetration speed increase. Efficiency factor increase is shown as a function of power on bottom-hole for different intermittency factor values is shown in Fig. 5.

Fig. 5
figure 5

Efficiency factor increased as a function of intermittency factor and power on bottom-hole

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As it can be seen from the data shown in Fig. 5, greater relative increase of penetration speed is achieved with lower power on bottom-hole values. At the same time, as it can be seen from Fig. 6, greater increase of absolute penetration speed corresponds to greater power on bottom-hole values.

Fig. 6
figure 6

Dependence of penetration speed on power on bottom-hole for different flushing modes

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It has been shown by Dreus et al. (Dreus et al. 2016a; Dreus et al. 2016b) that intermittent flushing provides bottom-hole surface temperature change during diamond core drilling. Therefore, as it has been shown in investigations ((Vettegrena et al. 2013)), conditions for intensified rock disintegration can be provided due to thermal weakening by intermittent impact of high and low temperatures.

Intermittency factor impact on efficiency criterion is presented in Fig. 7.

Fig. 7
figure 7

Intermittency factor kint impact on efficiency criterion on intermittent flushing for different power values

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The results shown in Fig. 7 confirm that frictional thermal power performs more efficiently at operating conditions assumed.

Some more advantages of transition to intermittent diamond drilling mode need to be mentioned. Average liquid flow under intermittent flushing is defined by expression

$$ {Q}_{av}=\frac{\underset{0}{\overset{T}{\int }}Q(t) dt}{T} $$
(14)

where Q(t) is a function of fluid flow variation under constant feed rate, t time, and T period (see Fig. 1). Therefore, transition to intermittent mode allows reducing of flushing liquid, and consequently, saving drilling mud. Furthermore, the pump pressure and the power to drive the pump reduces allows to save power.

Conclusions

The results of performed research show that diamond drilling using intermittent flushing proves to be an efficient method of penetration speed improvement.

The research showed that the penetration speed increased with the consideration of varying operating conditions. Penetration speed increase constituted 1.18 to 2.2 times, depending on intermittency factor value. Increase in intermittency factor provides increase in penetration speed.

It has been shown that improvement in rock destruction efficiency during diamond drilling using intermittent flushing has been higher in relative values at a lower power on bottom-hole values. In absolute values, penetration speed increase after transition to intermittent flushing mode is directly proportional to the power on bottom-hole level.

Utilization of intermittent flushing allows not only to increase penetration speed, but to improve other performance indicators, such as reduction of flow rate of flushing liquid at absorption and save power.