B737 Captain reported the airspeed decayed unnoticed during climb resulting in a buffet alert. Flight crew increased thrust and descended to regain airspeed.

Narrative

While climbing out on the ZZZZZX RNAV departure to FL370 we encountered light chop associated with cloud layers passing approximately FL180. I elected to use Speed Intervention to expedite climbing through the choppy altitudes selecting a speed of 280 kts. in the speed window. I forgot to deselect Speed Intervention (close the speed window) after climbing past the choppy area. As we continued to climb the Speed Intervention continued to attempt to hold the speed I had selected resulting in the indicated airspeed slowly decaying as we continued to climb. As we climbed through approximately 36;500 feet the airspeed had decayed into the top of the Amber Band and stimulated a FMC Buffet Alert. Upon seeing the Buffet Alert the First Officer immediately verbalized the alert. I noted the alert and saw the indicated airspeed in the Amber Band and immediately moved the thrust levers manually forward and gently pushed control column to slow the climb in an attempt to increase the airspeed. The push on control column manually overcame the Autopilot causing it to disconnect. The airspeed did not increase in a timely manner so I requested the First Officer request FL360 from Air Traffic Control (ATC). We momentarily leveled off; then descended to FL360 as ATC cleared us to FL360. I hand flew down to FL360 and the descent allowed the airspeed to increase in a timely manner out of the Amber Band. As I descended it came to me that I had not closed the Speed Intervention after the choppy area and that was the likely cause of the issue.