During the week of Sept. 19, U.S. drillers added oil rigs for a 12th week in the past 13 and were on track to add the most rigs in a quarter since crude prices plummeted two years ago, although the momentum has slowed as prices hold below $50 a barrel (bbl).

Drillers added two oil rigs in the week to Sept. 23, bringing the total rig count up to 418, the most since February but still below the 641 rigs seen a year ago, energy services firm Baker Hughes Inc said on Sept. 23.

The oil rig count plunged from a record high of 1,609 in October 2014 to a low of 316 in May after crude prices collapsed in the steepest drop in a generation due to a global oil glut. That decline continued through the first half of this year when drillers cut 206 rigs. So far during the current quarter, however, drillers have added or at least not removed any oil rigs for 13 weeks in a row, the longest streak of not cutting rigs since 2011.

With just one week to go in the quarter, oil rig additions over the past three months were on track to be the most since the first quarter of 2014 when drillers added 105 rigs. To date, the count has increased by 88 rigs so far this quarter. Continued growth in the rig count in the short-term, however, could be at risk if the small independent drillers, which accounted for about two-thirds of rig additions since May, pull back if prices remain low.

"As their cash flows ramped with oil moving from its February bottom to over $50, they redeployed their cash flows in the form of rigs," analysts at Evercore ISI, a U.S. investment banking advisory, said about the small independent drillers.

"With WTI [West Texas Intermediate crude] now oscillating in the mid-$40s and cash flows declining again due to the lower oil price, we wouldn't be surprised to see this group of operators let some rigs go," Evercore said.

U.S. WTI futures were below $45/bbl on Sept. 23 but were on track for a 5% hike for the week ahead of the meeting in Algeria during the week of Sept. 26, where the world's biggest producers are expected to discuss a deal to curb the global oil glut.

Looking forward, analysts said they still expect the rig count to rise in 2017 and 2018 over 2016. The rate of rise will not be as high as a few weeks ago because future oil price forecasts will hold in the $50/bbl to $60/bbl range. Futures for calendar year 2017 were trading around $48/bbl, while calendar year 2018 was about $51/bbl.

"Based on a lower assumed price deck of about $50-60 for the next two years, we are moderating our [E&P] capital spending and activity growth assumptions," analysts at Simmons & Co., energy specialists at U.S. investment bank Piper Jaffray, said in a note.

Simmons reduced its average total oil and natural gas rig count forecasts to 495 from 498 in 2016, to 647 from 704 in 2017 and to 857 from 981 in 2018. The combined number of oil and gas rigs active so far this year has averaged 480 rigs, according to Baker Hughes data. That compares with an average rig count of 978 in 2015.